The 1918-19 flu pandemic killed somewhere in the region of 50-100 million people. Whatever the figure it was almost certainly greater than the number of people killed during World War 1. Most experts agree that another epidemic is overdue, possibly not on the same scale, but devastating to at least our mental state nevertheless. On a slightly related note, expect to see a return to some old-fashioned diseases and conditions in the future — gout and rickets for example.