Sleeper viruses explain why HIV evolves more slowly between people than within them

HIV – the virus behind AIDS – is the most diverse of all viruses. Once it infects someone new, it mutates so rapidly that it can spawn a million genetically different strains in just a few months. This evolutionary onslaught overwhelms the host’s immune system, and creates big problems for any scientist trying to create a cure or a vaccine. By evolving so quickly, HIV turns itself into a million moving targets. But when HIV jumps from one individual to another, something odd happens. The virus still mutates at a breakneck speed, but it does so 2 to 6 times more slowly than within any single person. Unexpectedly, the virus seems to evolve faster in a single host, than in a population. There are three possible explanations for this puzzling trend, but Katrina Lythgoe and Christophe Fraser from Imperial College London think that only one is correct. They think that ...