Atlas of pathophysiology, 2 Edition

Part I - Central concepts

Infection

Infection is the invasion and multiplication of microorganisms in or on body tissue that cause signs, symptoms, and an immune response. Such reproduction injures the host by causing cell damage from toxins produced by the microorganisms or from intracellular multiplication or by competing with host metabolism. Infectious diseases range from relatively mild illnesses to debilitating and lethal conditions: from the common cold through chronic hepatitis to acquired immunodeficiency syndrome. The severity of the infection varies with the pathogenicity and number of the invading microorganisms and the strength of host defenses.

For infection to be transmitted, these factors must be present: causative agent, infectious reservoir with a portal of exit, mode of transmission, a portal of entry into the host, and a susceptible host.

Causes

Microorganisms that are responsible for infectious diseases include viruses, bacteria, fungi, mycoplasmas, rickettsia, chlamydia, spirochetes, and parasites.

Viruses

Viruses are subcellular organisms made up only of a ribonucleic acid (RNA) nucleus or a deoxyribonucleic acid (DNA) nucleus covered with proteins. They're the smallest known organisms, so tiny that only an electron microscope can make them visible. Independent of the host cells, viruses can't replicate. Rather, they invade a host cell and stimulate it to participate in forming additional virus particles. Some viruses destroy surrounding tissue and release toxins. Viruses lack the genes necessary for energy production. They depend on the ribosomes and nutrients of infected host cells for protein production. The estimated 400 viruses that infect humans are classified according to their size, shape, and means of transmission, such as respiratory, fecal, oral, and sexual.

Retroviruses are a unique type of virus that carry their genetic code in RNA rather than the more common carrier DNA. These RNA viruses contain the enzyme reverse transcriptase, which changes viral RNA into DNA. The host cell then incorporates the alien DNA into its own genetic material. The most notorious retrovirus known today is human immunodeficiency virus.

Bacteria

Bacteria are simple one-celled microorganisms with a cell wall that protects them from many of the defense mechanisms of the human body. Although they lack a nucleus, bacteria possess all the other mechanisms they need to survive and rapidly reproduce.

Bacteria can be classified according to shape—spherical cocci, rod-shaped bacilli, and spiral-shaped spirilla. They can also be classified according to their need for oxygen (aerobic or anaerobic), their mobility (motile or nonmotile), and their tendency to form protective capsules (encapsulated or nonencapsulated) or spores (sporulating or nonsporulating).

Bacteria damage body tissues by interfering with essential cell function or by releasing exotoxins or endotoxins, which cause cell damage.

Fungi

Fungi have rigid walls and nuclei that are enveloped by nuclear membranes. They occur as yeast (single-cell, oval-shaped organisms) or molds (organisms with hyphae, or branching filaments). Depending on the environment, some fungi may occur in both forms. Found almost everywhere on earth, fungi live on organic matter, in water and soil, on animals and plants, and on a wide variety of unlikely materials. They can live both inside and outside their host.

Mycoplasmas

Mycoplasmas are bacterialike organisms, the smallest of the cellular microbes that can live outside a host cell, although some may be parasitic. Lacking cell walls, they can assume many different shapes ranging from coccoid to filamentous. The lack of a cell wall makes them resistant to penicillin and other antibiotics that work by inhibiting cell wall synthesis.

Rickettsia

Rickettsia are small, gram-negative, aerobic bacterialike organisms that can cause life-threatening illness. They may be coccoid, rod-shaped, or irregularly shaped. Rickettsia require a host cell for replication. They have no cell wall, and their cell membranes are leaky; thus, they must live inside another, better protected cell. Rickettsia are transmitted by the bites of arthropod carriers, such as lice, fleas, and ticks, and through exposure to their waste products. Rickettsial infections that occur in the United States include Rocky Mountain spotted fever, typhus, and Q fever.

Chlamydia

Chlamydia are smaller than rickettsia and bacteria but larger than viruses. They depend on host cells for replication and are susceptible to antibiotics. Chlamydia are transmitted by direct contact such as occurs during sexual activity.

Spirochetes

Spirochetes are an atypical type of bacteria that have a helical shape, and the length is many times its width. They have filaments wrapped around the cell wall that propel the spirochete in a corkscrew motion. Spirochetes occur with Lyme disease and syphilis.

Parasites

Parasites are unicellular or multicellular organisms that live on or within another organism and obtain nourishment from the host. They take only the nutrients they need and usually don't kill their hosts. Examples of parasites that can produce an infection if they cause cellular damage to the host include helminths (such as pinworms, roundworms, and tapeworms) and arthropods (such as mites, fleas, and ticks). Helminths can infect the human gut; arthropods commonly cause skin and systemic disease.

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Risk factors

A healthy person can usually ward off infections with the body's own built-in defense mechanisms, which include:

·         intact skin

·         normal flora that inhabit the skin and various organs

·         lysozymes secreted by eyes, nasal passages, glands, stomach, and genitourinary organs

·         defensive structures such as the cilia that sweep foreign matter from the airways

·         a healthy immune system.

However, if an imbalance develops, the potential for infection increases. Risk factors for the development of infection include weakened defense mechanisms, environmental and developmental factors, and pathogen characteristics.

Weakened defense mechanisms

The body has many defense mechanisms for resisting entry and multiplication of both exogenous and endogenous microbes. However, a weakened immune system makes it easier for these pathogens to invade the body and launch an infectious disease. This weakened state is referred to as immunodeficiency or immunocompromise.

Impaired function of white blood cells (WBCs), as well as low levels of T cells and B cells, characterizes immunodeficiencies. An immunodeficiency may be congenital or acquired. Acquired immunodeficiency may result from infection, malnutrition, chronic stress, or pregnancy. Diabetes, renal failure, and cirrhosis can suppress the immune response, as can such drugs as corticosteroids and chemotherapy.

Regardless of cause, the result of immunodeficiency is the same. The body's ability to recognize and fight pathogens is impaired. People who are immunodeficient are more susceptible to all infections, are more acutely ill when they become infected, and require a much longer time period to heal.

Environmental factors

Other conditions that may weaken a person's immune defenses include poor hygiene, malnutrition, inadequate physical barriers, emotional and physical stressors, chronic diseases, medical and surgical treatments, and inadequate immunization.

Good hygiene promotes normal host defenses; poor hygiene increases the risk of infection. Unclean skin harbors microbes, offering an environment for them to colonize, and is more open to invasion. Frequent body washing removes surface microbes and maintains an intact barrier to infection, but it may damage the skin. To maintain skin integrity, lubricants and emollients may be used to prevent cracks and breaks.

The body requires a balanced diet to provide the nutrients, vitamins, and minerals needed for an effective immune system. Protein malnutrition inhibits the production of antibodies, without which the body is unable to mount an effective attack against microbe invasion. Malnutrition has been shown to have a direct relationship to the incidence of nosocomial infections.

Dust can facilitate transportation of pathogens. For example, dustborne spores of the fungus aspergillus transmit the infection. If the inhaled spores become established in the lungs, they're notoriously difficult to expel. Fortunately, most persons with intact immune systems can resist infection with aspergillus, which is usually dangerous only in the presence of severe immunosuppression.

Developmental factors

Extremely young and old people are at higher risk for infection. The immune system doesn't fully develop until about age 6 months. An infant exposed to an infectious agent usually develops an infection. The most common type of infection in toddlers affects the respiratory tract. When young children put toys and other objects in their mouths, they increase their exposure to a variety of pathogens.

Exposure to communicable diseases continues throughout childhood, as children progress from daycare facilities to schools. Skin diseases, such as impetigo, and lice infestation commonly pass from one child to the next at this age. Accidents are common in childhood as well, and broken or abraded skin opens the way for bacterial invasion. Lack of immunization also contributes to incidence of childhood diseases.

Advancing age, on the other hand, is associated with a declining immune system, partly as a result of decreasing thymus function. Chronic diseases, such as diabetes and atherosclerosis, can weaken defenses by impairing blood flow and nutrient delivery to body systems.

Pathogen characteristics

A microbe must be present in sufficient quantities to cause a disease in a healthy human. The number needed to cause a disease varies from one microbe to the next and from host to host and may be affected by the mode of transmission. The severity of an infection depends on several factors, including the microbe's pathogenicity, that is, the likelihood that it will cause pathogenic changes or disease. Factors that affect pathogenicity include:

·         specificity—the range of hosts to which a microbe is attracted (Some microbes may be attracted to a wide range of both humans and animals, while others select only human or only animal hosts.)

·         invasiveness (sometimes called infectivity)—ability of a microbe to invade and multiply in the host tissues (Some microbes can enter through intact skin; others can enter only if the skin or mucous membrane is broken. Some microbes produce enzymes that enhance their invasiveness.)

·         quantity—the number of microbes that succeed in invading and reproducing in the body

·         virulence—severity of the disease a pathogen can produce (Virulence can vary depending on the host defenses; any infection can be life-threatening in an immunodeficient patient. Infection with a pathogen known to be particularly virulent requires early diagnosis and treatment.)

·         toxigenicity (related to virulence)—potential to damage host tissues by producing and releasing toxins

·         adhesiveness—ability to attach to host tissue (Some pathogens secrete a sticky substance that helps them adhere to tissue while protecting them from the host's defense mechanisms.)

·         antigenicity—degree to which a pathogen can induce a specific immune response (Microbes that invade and localize in tissue initially stimulate a cellular response; those that disseminate quickly throughout the host's body generate an antibody response.)

·         viability—ability to survive outside its host. Most microbes can't live and multiply outside a reservoir.

Stages of infection

Development of infection usually proceeds through four stages. (See Stages of infection.)

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Pathophysiologic concepts

Clinical expressions of infectious disease vary depending on the pathogen involved and the body system affected. Most of the signs and symptoms result from host responses, which may be similar or extremely different from host to host. During the prodromal stage, a person will complain of some common, nonspecific signs and symptoms, such as fever, muscle aches, headache, and lethargy. In the acute stage, signs and symptoms that are more specific provide evidence of the microbe's target. However, some diseases produce no symptomes and are discovered only by laboratory tests.

The inflammatory response is a major reactive defense mechanism in the battle against infective agents. Inflammation may be the result of tissue injury, infection, or allergic reaction. Acute inflammation has two stages: vascular and cellular. In the vascular stage, arterioles at or near the site of the injury briefly constrict and then dilate, causing an increase in fluid pressure in the capillaries. The consequent movement of plasma into the interstitial space causes edema. At the same time, inflammatory cells release histamine and bradykinin, which further increase capillary permeability. Red blood cells and fluid flow into the interstitial space, contributing to edema. The extra fluid arriving in the inflamed area dilutes microbial toxins.

During the cellular stage of inflammation, WBCs and platelets move toward the damaged cells, and phagocytosis of the dead cells and microorganisms begins. Platelets control any excess bleeding in the area, and mast cells arriving at the site release heparin to maintain blood flow to the area.

Signs and symptoms

Acute inflammation is the body's immediate response to cell injury or cell death. The cardinal signs of inflammation include:

·         redness (rubor)—dilation of arterioles and increased circulation to the site; a localized blush caused by filling of previously empty or partially distended capillaries

·         heat (calor)—local vasodilatation, fluid leakage into the interstitial spaces, and increased blood flow to the area

·         pain (dolor)—pain receptors stimulated by swollen tissue, local pH changes, and chemicals excreted during the inflammatory process

·         edema (tumor)—local vasodilatation, leakage of fluid into interstitial spaces, and blockage of lymphatic drainage

·         loss of function (functio laesa) of a body part—primarily a result of edema and pain.

Clinical Tip

Localized infections produce a rapid inflammatory response with obvious signs and symptoms. Disseminated infections have a slow inflammatory response and take longer to identify and treat, thereby increasing morbidity and mortality.

Fever

Fever follows the introduction of an infectious agent. An elevated temperature helps fight an infection because many microorganisms are unable to survive in a hot environment. When the body temperature rises too high, body cells can be damaged, particularly those of the nervous system.

Diaphoresis (sweating) is the body's method of cooling itself and returning the temperature to normal for that individual. Artificial methods to reduce a slight fever can actually impede the body's defenses against infection.

Stages of infection

Stage 1

Incubation

·         Duration can range from instantaneous to several years.

·         Pathogen is replicating, and the infected person becomes contagious, thus capable of transmitting the disease.

Stage II

Prodromal stage

·         Host makes vague complaints of feeling unwell.

·         Host is still contagious.

Stage III

Acute illness

·         Microbes actively destroy host cells and affect specific host systems.

·         Patient recognizes which area of the body is affected.

·         Complaints are more specific.

Stage IV

Convalescence

·         Begins when the body's defense mechanisms have contained the microbes.

·         Damaged tissue is healing.

Leukocytosis

The body responds to the introduction of pathogens by increasing the number and types of circulating WBCs. This process is called leukocytosis. In the acute or early stage, the neutrophil count increases. Bone marrow begins to release immature leukocytes, because existing neutrophils can't meet the body's demand for defensive cells. The immature neutrophils (called bands in the differential WBC count) can't serve any defensive purpose.

As the acute phase comes under control and the damage is isolated, the cellular stage of the inflammatory process takes place. Neutrophils, monocytes, and macrophages begin the process of phagocytosis of dead tissue and bacteria. Neutrophils and monocytes are attracted to the site of infection by chemotaxis, and they identify the foreign antigen and attach to it. Then they engulf, kill, and degrade the microorganism that carries the antigen on its surface. Macrophages, a mature type of monocyte, arrive at the site later and remain in the area of inflammation longer than the other cells. Besides phagocytosis, macrophages play several other key roles at the site, such as preparing the area for healing and processing antigens for a cellular immune response. An elevated monocyte count is common during resolution of any injury and in chronic infections.

Chronic inflammation

An inflammatory reaction lasting longer than 2 weeks is referred to as chronic inflammation. It may follow an acute process. A poorly healed wound or an unresolved infection can lead to chronic inflammation. The body may encapsulate a pathogen that it can't destroy in order to isolate it. An example of such a pathogen is mycobacteria, one of the species that

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cause tuberculosis; encapsulated mycobacteria appear in X-rays as identifiable spots in the lungs. With chronic inflammation, permanent scarring and loss of tissue function can occur.

Diagnostic tests

Accurate assessment helps identify infectious diseases, appropriate treatment, and avoidable complications. It begins with obtaining the patient's complete medical history, performing a thorough physical examination, and performing or ordering appropriate diagnostic tests. Tests that can help identify and gauge the extent of infection include laboratory studies, radiographic tests, and scans.

Most commonly, the first test is a WBC count and a differential. An elevation in the overall number of WBCs is a positive result. The differential count is the relative number of each of five types of WBCs—neutrophils, eosinophils, basophils, lymphocytes, and monocytes. This test recognizes only that an immune response has been stimulated. Bacterial infection usually causes an elevation in the counts; viruses may cause no change or a decrease in normal WBC level.

Erythrocyte sedimentation rate may be done as a general test to reveal that an inflammatory process is occurring within the body.

To determine the causative agent, a stained smear from a specific body site is obtained. Stains that may be used to visualize the microorganism include:

·         Gram stain—identifies gram-negative or gram-positive bacteria

·         acid-fast stain—identifies mycobacteria and nocardia

·         silver stain—identifies fungi, legionella, and pneumocystis.

Although stains provide rapid and valuable diagnostic information, they only tentatively identify a pathogen. Confirmation requires culturing. Growth sufficient to identify the microbe may occur in as quickly as 8 hours or as long as several weeks, depending on how rapidly the microbe replicates. Types of cultures that may be ordered are blood, urine, sputum, throat, nasal, wound, skin, stool, and cerebrospinal fluid, but any body substance can be cultured.

A specimen obtained for culture must not be contaminated with any other substance. For example, a urine specimen must not contain debris from the perineum or vaginal area. If obtaining a clean urine specimen isn't possible, the patient must be catheterized to make sure that only the urine is being examined. Contaminated specimens may mislead and prolong treatment.

Additional tests that may be requested include magnetic resonance imaging to locate infection sites, chest X-rays to search the lungs for respiratory changes, and gallium scans to detect abscesses.

Treatment

Treatment for infections can vary widely. Vaccines may be administered to induce a primary immune response under conditions that won't cause disease. If infection occurs, treatment is tailored to the specific causative organism. Drug therapy should be used only when it's appropriate. Supportive therapy can play an important role in fighting infections.

·         Antibiotics work in a variety of ways, depending on the class of antibiotic. Their action is either bactericidal or bacteriostatic. Antibiotics may inhibit cell-wall synthesis, protein synthesis, bacterial metabolism, or nucleic acid synthesis or activity, or they may increase cell-membrane permeability.

·         Antifungal drugs destroy the invading microbe by increasing cell-membrane permeability. The antifungal binds sterols in the cell membrane, resulting in leakage of intracellular contents, such as potassium, sodium, and nutrients.

·         Antiviral drugs stop viral replication by interfering with DNA synthesis.

The overuse of antimicrobials has created widespread resistance to some specific drugs. Some pathogens that were once well controlled by medications are again surfacing with increased virulence. One such pathogen is that which is known to cause tuberculosis.

Some diseases, including most viral infections, don't respond to available drugs. Supportive care is the only recourse while the host defenses repel the invader. To help the body fight an infection, the patient should:

·         use universal precautions to avoid spreading the infection

·         drink plenty of fluids

·         get plenty of rest

·         avoid people who may have other illnesses

·         take only over-the-counter medications appropriate for his symptoms, with full knowledge about dosage, actions, and possible adverse effects or reactions

·         follow the health care provider's orders for taking prescription drugs, and be sure to complete the medication as ordered and not share the prescription with others.

Disorders

See Common infectious disorders, which describes a variety of infectious disorders.

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Common infectious disorders

Disorder

Characteristics

Bacterial infections

Anthrax

Bacterial infection characterized as cutaneous, inhalational, or intestinal.

·         Diagnosis confirmed by isolation of Bacillus anthracis from cultures of blood, skin, lesions, or sputum.

·         Signs and symptoms directly related to the location of infection. Cutaneous anthrax is characterized by a small, elevated, itchy lesion, which develops into a vesicle and then a painless ulcer, along with enlarged lymph glands. Inhalational anthrax is characterized by flulike symptoms initially, followed by severe respiratory difficulty and shock. With intestinal anthrax, fever, nausea, vomiting, and decreased appetite occur, which progress to abdominal pain, hematemesis, and severe diarrhea.

Chlamydia

Sexually transmitted infection caused by Chlamydia trachomatis.

·         Disease pattern depends on the individual infected and the site of infection.

Conjunctivitis

Bacterial or viral infection of the conjunctiva of the eye.

·         Culture from the conjunctiva identifies the causative organism.

·         Associated with hyperemia of the eye, discharge, tearing, pain, and photophobia.

Gonorrhea

Sexually transmitted infection caused by Neisseria gonorrhoeae, a gram-negative, oxidase-positive diplococcus.

·         After exposure, epithelial cells at infection site become infected and the disease begins to spread locally.

·         Disease pattern depends on the individual infected and the site of infection.

Listeriosis

Infection caused by weakly hemolytic, gram-positive bacillus Listeria monocytogenes.

·         Primary method of person-to-person transmission is neonatal infection in utero or during passage through an infected birth canal.

·         Disease may cause abortion, premature delivery, stillbirth, or organ abscesses in fetuses.

·         Neonates may have tense fontanels due to meningitis, be irritabile or lethargic, have seizures, or be comatose.

Lyme disease

Infection caused by spirochets Borrelia burgdorferi.

·         Transmitted by ixodid tick, which injects spirochete-laden saliva into the bloodstream or deposits fecal matter on the skin.

·         After 3 to 32 days, spirochetes migrate outward, typically causing a ringlike rash, called erythema chronicum migrans.

·         Spirochetes disseminate to other skin sites or organs through the bloodstream or lymphatic system.

·         Spirochetes may survive for years in joints, or they may die after triggering an inflammatory response in the host.

·         As infection progresses through three stages, neurologic symptoms and impairment worsen.

Meningitis

Meningeal inflammation caused by bacteria, viruses, protozoa, or fungi. The most common types are bacterial and viral.

·         Disease occurs when infecting organisms enter the subarachnoid space and cause an inflammatory response. The organisms gain access to the cerebrospinal fluid, where they cause irritation of the tissues bathed by the fluid.

·         Characteristic signs include fever, chills, headache, nuchal rigidity, vomiting, photophobia, lethargy, coma, positive Brudzinki's and Kernig's signs, increased deep tendon reflexes, widened pulse pressure, bradycardia, and rash.

Otitis media

Inflammation of the middle ear caused by a bacterial infection.

·         Disease is commonly accompanied by a viral upper respiratory infection.

·         Viral symptoms occur, generally followed by ear pain.

Peritonitis

Acute or chronic inflammation of the peritoneum caused by bacterial invasion.

·         Onset commonly sudden, with severe and diffuse abdominal pain.

·         Pain intensifies and localizes in the region of infection.

Pertussis (whooping cough)

Highly contagious respiratory infection usually caused by the nonmotile, gram-negative coccobacillus Bordetella pertussis and, occasionally, by the related similar bacteria B. parapertussis or B. bronchiseptica.

·         Transmitted by direct inhalation of contaminated droplets from a patient in an acute stage. It may also spread indirectly though soiled linen and other articles contaminated by respiratory secretions.

·         After approximately 7 to 10 days, B. pertussis enters the tracheobronchial mucosa, where it produces progressively tenacious mucus.

·         Known for its associated spasmodic cough, characteristically ending in a loud, crowing inspiratory whoop. Complications include apnea, hypoxia, seizures, pneumonia, encephalopathy, and death.

Pneumonia

Infection of the lung parenchyma that's bacterial, fungal, or protozoal in origin.

·         The lower respiratory tract can be exposed to pathogens by inhalation, aspiration, vascular dissemination, or direct contact with contaminated equipment. When inside, the pathogen begins to colonize and infection develops.

·         Bacterial infection initially triggers alveolar inflammation and edema, which produces an area of low ventilation with normal perfusion. Capillaries become engorged with blood, causing stasis. As alveolocapillary membranes break down, alveoli fill with blood and exudates, causing atelectasis, or lung collapse.

Salmonellosis

Disease caused by a serotype of the genus Salmonella, a member of the Enterobacteriaceae family.

·         Most common species of Salmonella include S. typhi, which causes typhoid fever; S. enteritidis, which causes enterocolitis; and S. choleraesuis, which causes bacteremia.

·         Nontyphoidal salmonellosis usually follows ingestion of contaminated dry milk, chocolate bars, pharmaceuticals of animal origin, or contaminated or inadequately processed foods, especially eggs and poultry.

·         Characteristic symptoms include fever, abdominal pain or cramps, and severe diarrhea with enterocolitis.

Shigellosis

Acute intestinal infection caused by the bacteria shigella, a member of the Enterobacteriaceae family. It's a short, nonmotile, gram-negative rod.

·         Transmission occurs primarily through the fecal-oral route.

·         After an incubation period of 1 to 4 days, shigella organisms invade the intestinal mucosa and cause inflammation. Symptoms can range from watery stools to fever, cramps, and stools with pus, mucus, or blood.

Tetanus

Acute exotoxin-mediated infection caused by the anaerobic, spore-forming, gram-positive bacillus Clostridium tetani.

·         Transmission occurs through a puncture wound that's contaminated by soil, dust, or animal excreta containing C. tetani, or by way of burns and minor wounds.

·         After C. tetani enters the body, it causes local infection and tissue necrosis. It also produces toxins that then enter the bloodstream and lymphatics and eventually spread to central nervous system (CNS) tissue.

·         Disease is characterized by marked muscle hypertonicity, hyperactive deep tendon reflexes, and painful, involuntary muscle contractions. Severe muscle spasms can last up to 7 days.

Toxic shock syndrome (TSS)

Acute bacterial infection caused by toxin-producing, penicillin-resistant strains of Staphylococcus aureus, such as TSS toxin-1 or staphylococcal enterotoxins B and C. It can also be caused by Streptococcus pyogenes.

·         Menstrual TSS is associated with tampon use.

·         Nonmenstrual TSS is associated with infections, such as abcesses, osteomyelitis, pneumonia, endocarditis, bacteremia, and postsurgical infections.

·         Signs and symptoms include fever, hypotension, renal failure, and multisystem involvement.

Tuberculosis

Infectious disease transmitted by inhaling Mycobacterium tuberculosis, an acid-fast bacillus, from an infected person.

·         Bacilli are deposited in the lungs, the immune system responds by sending leukocytes, and inflammation results. After a few days, leukocytes are replaced by macrophages. Bacilli are then ingested by the macrophages and carried off by the lymphatics to the lymph nodes. Macrophages that infest the bacilli fuse to form ephithelioid cell tubercles, tiny nodules surrounded by lymphocytes.

·         Caseous necrosis develops in the lesion, and scar tissue encapsulates the tubercle. The organism may be killed in the process.

·         If the tubercles and inflamed nodes rupture, the infection contaminates the surrounding tissue and may spread through the blood and lymphatic circulation to distant sites.

Urinary tract infection

Infection most commonly caused by enteric gram-negative bacilli.

·         Results from microorganisms entering the urethra and then ascending into the bladder.

·         Commonly causes urgency, frequency, and dysuria.

Viral infections

Avian influenza

An influenza A virus that typically infects birds.

·         Reported symptoms in humans include fever, cough, sore throat, and muscle aches.

·         Virus could progress to eye infections, pneumonia, and acute respiratory distress.

Viral infections

Cytomegalovirus infection

A DNA virus that's a member of the herpes virus group.

·         Transmission can occur horizontally (person-to-person contact with secretions), vertically (mother to neonate), or through blood transfusions.

·         The virus spreads through the body in lymphocytes or mononuclear cells to the lungs, liver, GI tract, eyes, and CNS, where it commonly produces inflammatory reactions.

Herpes simplex virus (HSV)

HSV is an enveloped, double-stranded DNA virus that causes both herpes simplex type 1 and type 2.

·         Type 1 HSV is transmitted via oral and respiratory secretions; type 2 HSV is transmitted via sexual contact.

·         During exposure, the virus fuses to the host cell membrane and releases proteins, turning off the host cell's protein production or synthesis. The virus then replicates and synthesizes structural proteins. The virus pushes its nucleocapsid (protein coat and nucleic acid) into the cytoplasm of the host cell and releases the viral DNA. Complete virus particles capable of surviving and infecting a living cell are transported to the cell's surface.

·         Characteristic painful, vesicular lesions are usually observed at the site of initial infection.

Herpes zoster

Caused by a reactivation of varicella-zoster virus that has been lying dormant in the cerebral ganglia or the ganglia of posterior nerve roots.

·         Small, painful, red, nodular skin lesions develop on areas along nerve paths.

·         Lesions change to vesicles filled with pus or fluid.

Human immunodeficiency virus (HIV) infection

An RNA retrovirus that causes acquired immunodeficiency deficiency syndrome (AIDS).

·         Virus passes from one person to another through blood-to-blood and sexual contact. In addition, an infected pregnant woman can pass HIV to her baby during pregnancy or delivery as well as through breast-feeding.

·         Most people with HIV infection develop AIDS; however, current combination drug therapy in conjunction with treatment and prophylaxis of common opportunistic infections can delay the natural progression and prolong survival.

Infectious mononucleosis

Viral illness caused by the Epstein-Barr virus, a B-lymphotropic herpes virus.

·         Most cases spread by the oropharyngeal route, but transmission by blood transfusion or during cardiac surgery is also possible.

·         The virus invades the B cells of the oropharyngeal lymphoid tissues and then replicates.

·         Dying B cells release the virus into the blood, causing fever and other symptoms. During this period, antiviral antibodies appear and the virus disappears from the blood, lodging mainly in the parotid gland.

Monkeypox

Rare disease caused by the monkeypox virus, which belongs to the orthopoxvirus group.

·         In humans, monkeypox causes swollen lymph nodes, fever, headache, muscle aches, backache, exhaustion, and a papular rash with lesions that eventually crust and fall off.

Mumps

Acute viral disease caused by an RNA virus classified as Rubulavirus in the Paramyxoviridae family.

·         Virus is transmitted by droplets or by direct contact.

·         Characterized by enlargement and tenderness of parotid gland and swelling of other salivary glands.

Rabies

Rapidly progressive infection of the CNS caused by an RNA virus in the Rhabdoviridae family.

·         Transmitted by the bite of an infected animal through the skin or mucous membranes or, occasionally, in airborne droplets or infected tissue.

·         The rabies virus begins replicating in the striated muscle cells at the bite site.

·         The virus spreads along the nerve pathways to the spinal cord and brain, where it replicates again.

·         The virus moves through the nerves into other tissues, including into the salivary glands.

Respiratory syncytial virus

Infection of the respiratory tract caused by an enveloped RNA paramyxovirus.

·         The organism is transmitted from person to person by respiratory secretions or by touching contaminated surfaces.

·         Bronchiolitis or pneumonia ensues and, in severe cases, may damage the bronchiolar epithelium.

·         Interalveolar thickening and filling of alveolar spaces with fluid may occur.

·         The virus is more common in winter and early spring.

Rubella

An enveloped positive-stranded RNA virus classified as a rubivirus in the Togaviridae family.

·         Transmitted through contact with the blood, urine, stool, or nasopharyngeal secretions of an infected person. It can also be transmitted transplacentally.

·         The virus replicates first in the respiratory tract and then spreads through the bloodstream.

·         Characteristic maculopapular rash usually begins on the face and then spreads rapidly.

Rubeola

Acute, highly contagious paramyxovirus infection that's spread by direct contact or by contaminated airborne respiratory droplets.

·         Portal of entry is the upper respiratory tract.

·         Characterized by Koplik's spots, a pruritic macular rash that becomes papular and erythematous.

Smallpox

Acute contagious virus caused by the variola virus, a member of the Orthopoxvirus family.

·         Transmitted from person to person by infected aerosols and air droplets.

·         The incubation period, which is usually 12 to 14 days, is followed by the sudden onset of influenzalike symptoms including fever, malaise, headache, and severe back pain.

·         Two to 3 days later, the patient may feel better, however, the characteristic rash appears, first on the face, hands, and forearms and then after a few days progressing to the trunk. Lesions also develop in the mucous membranes of the nose and mouth, then ulcerate and release large amounts of virus into the mouth and throat.

Varicella (chickenpox)

Common, highly contagious exanthem caused by the varicella-zoster virus, a member of the herpes virus family.

·         Transmitted by respiratory droplets or contact with vesicles. In utero infection is also possible.

·         Characterized by a pruritic rash of small, erythematous macules that progresses to papules and then to clear vesicles on an erythematous base.

Viral pneumonia

Lung infection caused by any one of a variety of viruses, transmitted through contact with an infected individual.

·         The virus first attacks bronchiolar epithelial cells, causing interstitial inflammation and desquamation.

·         Virus invades bronchial mucous glands and goblet cells and then spreads to the alveoli, which fill with blood and fluid. In advanced infection, a hyaline membrane may form.

Fungal infection

Histoplasmosis

Fungal infection caused by Histoplasma capsulatum, a dimorphic fungus.

·         Transmitted through inhalation of H. capsulatum spores or invasion of spores after minor skin trauma.

·         Initially, infected person may be asymptomatic or have symptoms of mild respiratory illness, progressing into more severe illness affecting several organ systems.

Protozoal infections

Toxoplasmosis

Infection caused by the intracellular parasite Toxoplasma gondii, which affects both birds and mammals.

·         Transmitted to humans by ingestion of tissue cysts in raw or undercooked meat or by fecal oral contamination from infected cats. Direct transmission can also occur during blood transfusions, organ transplants, or bone marrow transplants.

·         When tissue cysts are ingested, parasites are released, which quickly invade and multiply within the GI tract. The parasitic cells rupture the invaded host cell and then disseminate to the CNS, lymphatic tissue, skeletal muscle, myocardium, retina, and placenta.

·         As the parasites replicate and invade adjoining cells, cell death and focal necrosis occur, surrounded by an acute inflammatory response, which are the hallmarks of this infection.

·         After the cysts reach maturity, the inflammatory process is undetectable and the cysts remain latent within the brain until they rupture.

·         In the normal host, the immune response checks the infection, but this isn't so with immunocompromised or fetal hosts. In these patients, focal destruction results in necrotizing encephalitis, pneumonia, myocarditis, and organ failure.

Trichinosis

Infection caused by the parasite Trichinella spiralis and transmitted through ingestion of uncooked or undercooked meat that contains encysted larvae.

·         After gastric juices free the larva from the cyst capsule, it reaches sexual maturity in a few days. The female roundworm burrows into the intestinal mucosa and reproduces.

·         Larvae then travel through the lymphatic system and the bloodstream. They become embedded as cysts in striated muscle, especially in the diaphragm, chest, arms, and legs.