Epidemiologists are braced for the big one: the strain of flu that rivals the pandemic of 1918-1919, which killed at least 20 million people worldwide. In recent years, we have experienced scares with a host of new influenza viruses: bird flu, swine flu, Spanish flu, Hong Kong flu, H5N1, and most recently, H5N7. While these diseases appear to emerge from thin air, in fact, human activity is driving them. And the problem is not just flu, but a series...