One way our bodies resist disease is by producing a variety of chemicals called “antibodies,” which destroy the virus or bacterium causing the disease. Antibodies have to be selective to avoid destroying the wrong cells, so each type of antibody recognizes and attacks only a specific virus or bacterium. And that means that each new disease requires a new type of antibody. But in fighting disease, evolution sometimes works against us because the faster an organism multiplies, the faster it adapts to changes in its environment. For bacteria, the appearance of a new antibody means a big change in its environment, but unfortunately it’s a change the bacteria often adapt to. Since a single bacterium can multiply a thousand-fold in three hours, there’s a good chance that some of those offspring will vary from the rest in ways that make them unrecognizable to the antibody. Those bacteria will then produce millions more like themselves.