Chapter 6: Viruses—Basic Concepts

INTRODUCTION

A virus is a set of genes, composed of either DNA or RNA, packaged in a protein- containing coat called a capsid. Some viruses also have an outer lipid bilayer membrane external to the coat called an envelope. The resulting complete virus particle is called a virion. Viruses have an obligate requirement for intracellular growth and a heavy dependence on host cell structural and metabolic components. Therefore, viruses are also referred to as obligate intracellular parasites. Viruses do not have a nucleus, cytoplasm, mitochondria, or other cell organelles. Viruses that infect humans are called human viruses, but are considered along with the general class of animal viruses; viruses that infect bacteria are referred to as bacteriophages (phages for short), and viruses that infect plants are called plant viruses.

Virus reproduction requires that a virus particle infect an appropriate host cell and program the cellular machinery to synthesize the viral components required for the assembly of new virions, generally termed progeny virions or daughter viruses. The infected host cell may produce hundreds to hundreds of thousands of new virions, usually accompanied by cell death. Tissue damage as a result of cell death accounts for the pathology of many viral diseases in humans. Many of these viruses cause acute viral infection followed by viral clearance. In some cases, the infected cells survive, resulting in persistent virus production and a chronic infection that can remain asymptomatic, produce a chronic disease state, or lead to relapse of an infection.

Some viruses following acute infection cause a chronic infection with little to no symptoms but damage accumulates over time

In some circumstances, a virus fails to reproduce itself and, instead, enters a latent state (called lysogeny in the case of bacteriophages), from which there is the potential for reactivation at a later time. A possible consequence of the presence of viral genome in a latent state is a new genotype for the cell. Some determinants of bacterial virulence and some malignancies of animal cells are examples of the genetic effects of latent viruses. Apparently, vertebrates have had to coexist with viruses for a long time because they have evolved the special nonspecific interferon system, which operates in conjunction with the highly specific immune system to combat virus infections.

Two classes of infectious agents exist that are structurally simpler than viruses, namely, viroids and prions. Viroids are infectious circular RNA molecules that lack protein shells; they are responsible for a variety of plant diseases. Prions,...