The fight against infectious disease has been one of the most important driving forces in human evolution.So you might expect that evolution has optimised our immune system, and there won't be much low-hanging fruit for improving this with science.But bacteria and viruses evolve so quickly that evolution is constantly racing just to keep up. It doesn't have time to optimise perfectly.Case in point: Our primate ancestors used to have built-in genetic defence against HIV-like viruses. But at some