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 the ancestral strain – the one that kicked off someone’s infection – is more likely to spread to other people than its millions of descendants.

The progeny of the ancestral virus quickly evolve to avoid their host’s immune system and reproduce as