The surface of a virion contains many copies of one type of protein that binds specifically to multiple copies of a receptor protein on a host-cell surface. This interaction determines the host range—the group of cell types that a virus can infect—and begins the infection process. Most viruses have a rather limited host range.

A virus that infects only bacteria is called a bacteriophage, or simply a phage. Viruses that infect animal or plant cells are referred to generally as animal viruses or plant viruses. A few viruses can grow in both plants or animals and the insects that feed on them. The highly mobile insects serve as vectors for transferring such viruses between susceptible animal or plant hosts. Wide host ranges are also characteristic of some strictly animal viruses, such as vesicular stomatitis virus, which grows in insect vectors and in many different types of mammals. Most animal viruses, however, do not cross phyla, and some (e.g., poliovirus) infect only closely related species, such as primates. The host-cell range of some animal viruses is further restricted to a limited number of cell types because only those cells have surface receptors to which the virions can attach. One example is poliovirus, which infects only cells in the intestine and, unfortunately for its host, motor neurons in the spinal cord, causing paralysis. Another is HIV-1, discussed further below, which infects cells called CD4⁺ T lymphocytes that are essential for the immune response (see Chapter 23) as well as certain neurons and other cells of the central nervous system called glial cells.

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