A virus is a small piece of genetic information (usually in the form of RNA or DNA) in a “carrying case”—a protective protein coating called a capsid. When a virus infects a cell, the viral nucleic acid instructs the host cell to produce copies of itself using its own machinery. Viruses can infect plants, animals, bacteria and even fungi. They can also infect people and cause diseases such as polio, the common cold, measles and smallpox.
Viruses are very, very small—thousands of times smaller than the cells in your body. They can be seen only under a microscope, and are usually only 20 to 400 nanometers in size. They vary in shape, and the type of viruses is classified by whether they are icosahedral, helical or spiky; by the presence or absence of an envelope around them; by their genome and nucleic acid sequence; and by how they are transmitted from person to person.
The structure of a virus is determined by the proteins that make up its capsid. In many cases, the capsid protein has a regular geometric shape—an icosahedron (see Figure 2.1). This shape results from interactions between the proteins that make up the capsid.
Some scientists argue that viruses are not living things because they can’t reproduce on their own and must harness the machinery of a host cell to make more of themselves. Other scientists point out that, because viruses are made of the same building blocks as life—DNA and RNA—and can exist in two states, alive and not living, they are at the edge of what it means to be alive.