Every day we encounter lots of illness-inducing viruses and bacteria. Yet some people seem to never get sick. They have extra protection hard-wired into their genes, like people born with a specific mutation in the CCR5 gene are naturally resistant to HIV infection. The CCR5 protein acts like a door that lets HIV into the cells; without it, the virus is locked out. Researchers traced the mutation back 700 years to the time of the Black Death and suggested that those with the mutation were more likely to survive the plague and pass on their genes to current generations. So let´s see how some species naturally evolve in such a way that they become a lot more resilient than we are.