CHAPTER 1

INTRODUCTION TO VIRUSES

I. OVERVIEW. Viruses areobligate intracellular parasites. Unlike all other organisms, they are not “alive” because they are metabolically inactive on their own. They are also not “dead” because they can metabolize and reproduce when associated with a host cell. Instead, they are referred to as being “active” or “inactive.” Viruses are difficult to study because of their minuscule size, but they are even more abundant than bacteria. Most are part of normal environmental or human flora but some viruses are medically relevant and can cause infections that fall anywhere on the spectrum, from asymptomatic to fulminant. Several factors affect the pathogenicity of a virus.

1. Virus-specific factors

• Virulence: Some viruses are more virulent than others. For example, rabies virus is highly pathogenic, while torque teno virus does not cause disease despite being ubiquitous in normal flora.
• Persistence: Some viruses, like herpesviruses, cause mild disease but infect humans for life.
• Indirect effects: Viruses like bacteriophages only infect bacteria but are still indirectly pathogenic to humans. For example,Corynebacterium diphtheriae does not generally cause clinically significant disease unless it is infected with the bacteriophage containing the diphtheria toxin gene.

2. Host-specific factors: Hereditary genetic mutations can allow viruses that are weakly pathogenic to cause significant disease. For example, a specific mutation in CCR5 (a host cell receptor for HIV) has been shown to prevent infection with this virus. On the other hand, other mutations can result in overgrowth of viruses. For example, human papillomavirus 2 (HPV2) typically causes benign warts, but individuals with genetic defects in cell-mediated immunity can demonstrate uncontrolled giant warty overgrowths (“tree man” disease).

3. Immunosuppression: Immunosuppressive drugs, virus-induced immunosuppression, and even pregnancy are all instances in which the immune system is depressed. This can leave patients vulnerable to unique viral infections.

II. VIRUS STRUCTURE. The structure of a virus defines its life cycle, mechanism of pathogenicity, and how it is detected by laboratory assays. A virus particle, orvirion, is composed of nucleic acid surrounded by a protective protein coat called a capsid. Together, the nucleic acid and capsid are called thenucleocapsid.

1. Capsids: Occur in three main shapes (Fig. 1.1).

Figure 1.1.Viral capsids come in three main shapes.

• Polyhedral capsids have multiple flat sides that form a rigid shell around the viral nucleic acid. Viruses with lots of flat sides can appear round. Viruses with these kinds of capsids are highly regular and have a rigid shape and size. Most DNA viruses are polyhedral.
• Helical capsids wrap proteins around the strand of nucleic acid to form a spiral, elongated nucleocapsid. These viruses tend to be more variable in size and shape. Most RNA viruses are helical.
• Complex capsids have other shapes or are a combination of helical and polyhedral capsids.

2. Envelope