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.
Evolution Working Against Us
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.
DNA v. RNA
Some viruses, including HIV, which is responsible for AIDS, evolve even faster than bacteria. Unlike bacteria (as well as plants and animals), which carry their genetic information in the form of DNA, these viruses carry their genetic information in a similar molecule called RNA.
When RNA is copied to make a new virus, the process is less accurate and far more mutations occur than when DNA is copied. That means more variety among the offspring and more chances that a few of them will escape the antibodies.
In fact, viruses evolve so quickly that the same virus in two individuals often evolves into two different strains.
All living organisms adapt to changes in their environment but RNA viruses, like HIV, adapt much faster because they produce more offspring with more variation.