Fire and Knowledge

Almost as soon as [HIV] starts multiplying, our immune system starts recognizing the infected white blood cells and destroying them, wiping out the viruses in the process. But despite the immune system’s ability to kill HIV by the billions every day, HIV can survive these attacks for years. The secret to its longevity is its ability to evolve. The enzymes that HIV uses to make new copies of its genes are very sloppy, making one or two mistakes on average every time they duplicate the virus’s genome. Among the many mutants that spring up, a few strains will turn out to be hard for the immune system to recognize. Because HIV replicates so quickly, these resistant viruses quickly become the dominate strains in a person’s body. It takes time for our immune system to shift its attack toward the new strain, and once it does, the viruses evolve even newer forms that escape the immune system yet again.

–Carl Zimmer, Evolution: The Triumph of an Idea, p. 218