It’s worth taking a moment to consider viruses. A virus is an agent that infects cells. It is smaller and simpler than cells. Why, then, aren’t viruses the smallest unit of life? We just considered three essential features of cells—the capacity to store and transmit information, a membrane that selectively controls movement in and out, and the ability to harness energy from the environment. Viruses have a stable archive of genetic information, which can be RNA or DNA, surrounded by a protein coat and sometimes a lipid envelope. But viruses cannot harness energy from the environment. Therefore, on their own viruses cannot read and use the information contained in their genetic material, nor can they regulate the passage of substances across their protein coats or lipid envelopes the way that cells do. To replicate, they require a cell.

A virus infects a cell by binding to the cell’s surface, inserting its genetic material into the cell, and, in most cases, using the cellular machinery to produce more viruses. In this way, it is often said that a virus “hijacks” a cell. The infected cell may produce more viruses, sometimes by lysis, or breakage, of the cell, and the new viruses can then infect more cells. In some cases, the genetic material of the virus integrates into the DNA of the host cell.

We discuss viruses many times throughout the book. Each species of Bacteria, Archaea, and Eukarya is susceptible to many types of virus that are specialized to infect its cells. Several hundred types of virus are known to infect humans, and the catalog is still incomplete. Useful tools in biological research, viruses have provided a model system for many problems in biology, including how genes are turned on and off and how cancer develops.