Introduction to the Principles of Virology

This chapter defines what a virus is, how it replicates, and how it causes diseases. Peter Medawar, a biologist awarded the Nobel Prize for Medicine and Physiology in 1960, defined viruses as a piece of nucleic acid surrounded by bad news. Viruses cannot multiply until they invade a living cell. However, viruses can enter all cellular forms of life from plants and animals to bacteria, fungi, and protozoa. As opposed to plants and animals, which are made up of cells, viruses lack cell walls and are therefore obligatory parasites that depend for replication on the cells they infect. The attachment or binding of a viral protein to a cell receptor is the first step that initiates infection of a cell. The type of cells with such receptors and/or with the ability to replicate a given virus often determines the severity of illness that a virus can cause, the distribution of areas in the body that can be affected, and the host’s potential for recovery.