The other infectious agents described in this book, namely, bacteria, fungi, protozoa, and worms, are either single cells or composed of many cells. Cells are capable of independent replication, can synthesize their own energy and proteins, and can be seen in the light microscope. In contrast, viruses are not cells; they are not capable of independent replication, can synthesize neither their own energy nor their own proteins, and are too small to be seen in the light microscope.
Viruses are characterized by the following features:
(1) Viruses are particles composed of an internal core containing either DNA or RNA (but not both) covered by a protective protein coat. Some viruses have an outer lipoprotein membrane, called an envelope, external to the coat. Viruses do not have a nucleus, cytoplasm, mitochondria, or ribosomes. Cells, both prokaryotic and eukaryotic cells, have both DNA and RNA. Eukaryotic cells, such as fungal, protozoal, and human cells, have a nucleus, cytoplasm, mitochondria, and ribosomes. Prokaryotic cells, such as bacteria, are not divided into nucleus and cytoplasm and do not have mitochondria but do have ribosomes; therefore, they can synthesize their own proteins.
(2) Viruses must reproduce (replicate) within cells, because they cannot generate energy or synthesize proteins. Because they can reproduce only within cells, viruses are obligate intracellular parasites. (The only bacteria that are obligate intracellular parasites are chlamydiae and rickettsiae. They cannot synthesize sufficient energy to replicate independently.)
(3) Viruses replicate in a manner different from that of cells (i.e., viruses do not undergo binary fission or mitosis). One virus can replicate to produce hundreds of progeny viruses, whereas one cell divides to produce only two daughter cells.
Table III–1 compares some of the attributes of viruses and cells.
TABLE III–1 Comparison of Viruses and Cells