Although human DNA is 98 per cent similar to that of the chimpanzee, the infections we catch are 80 per cent different. Most are new acquisitions that we have picked up as humans spread across the world. In fact, pandemic infections like smallpox and influenza only date from the last 12,000 years or so after we formed settled farming communities and later developed large colonies known as cities. Does the history of infectious diseases help to predict future epidemics?