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Introduction to Viruses

Author(s): Christine Herrmann, PhD

What Organisms and Host Cells Do Viruses Infect?

A virus must infect (invade cells of) a living organism in order to reproduce. The organism that a virus infects is said to be the “host,” and the variety of organisms that a virus can infect is termed the “host range.” Although viruses infect many kinds of living organisms, each particular virus usually can infect only a narrow range of organisms, and sometimes only a single species. In such cases, the virus is said to have a “narrow host range”. Smallpox, for example, only is capable of infecting humans and no other species. This property of smallpox facilitated its eradication.

In addition, within a specific organism, viruses usually infect only a particular cell type or a limited number of cell types. Therefore, viruses that cause respiratory disease usually infect cells only in the respiratory tract. Viruses infect only certain cell types because a virus must bind to a certain receptor on the surface of the host cell, which allows the virus to enter into the cell. Using a “lock-and-key” analogy, the virus has a specially-shaped “key” that will fit only into a particular “lock”—the receptor.  Different cell types express different genes, so the proteins present on the outsides of their cells will be different. A lung cell, therefore, will have different proteins on its surface than a blood cell. HIV, for example, only will enter cells that have a surface protein molecule called CD4. These molecules are found only on white blood cells. Thus, HIV will only infect white blood cells and not lung cells or other cell types.

Sometimes, a virus can mutate and change its host range. This appears to be happening with the avian flu virus (influenza A/H5N1) currently circulating. At first, the flu virus could infect only birds, but changes in the virus genome have allowed it to infect humans. So now, its host range has expanded. The change in host range of a virus can be one factor in the emergence of a new disease.

Funded by the following grant(s)

Video and transcript courtesy of Wah Chiu, PhD, National Center for Macromolecular Imaging at Baylor College of Medicine. Funding for the video provided by NCMI, NIH.