HIV seems to have learned a thing or two from Proteus. It constantly changes form, eluding the immune system much the way that mythological sea creature evaded Menelaus. Although a person typically gets infected with a single strain of the virus, after a decade of infection two HIV viruses in the body may differ by as much as 10 percent — a greater difference than that between the key regions of mouse and human DNA. So before the immune system has managed to grasp the first form of HIV, the virus has changed form enough to become unrecognizable.