RM: It could be either. It would be simplest, of course, if it were one disease. But it strikes me that while it might have been one disease in North America, it was almost sure to have been quite another that took down the giant lemurs of Madagascar less than 2,000 years ago. This would scarcely be remarkable. The picture that I get just from what we know about human disease and the introduction of new diseases into human populations is that literally anything can take you down. You may know that there¿s a huge controversy right now about disease being purposely introduced into Yanomam¿ country in Amazonia. There was a major epidemic of measles there in the '60s, and people died of it. Yet for European-derived populations, dying from measles is extremely rare. So there¿s no reason at all to think that even garden-variety diseases that are prevalent today could not have that killer impact on na¿ve populations. Alternatively, it is also plausible to think that slight genetic changes in relatively benign microbes might render them lethal. For a good example we need go no further than the type A flu that went global in 1918 at the end of the First World War. This was a true killer plague¿the worst in recent times¿and it caused the death of between 20 and 40 million people in about a year and a half. Yet this novel flu evidently gained its lethality through a couple of substitutions in a couple of its genes. The point is these things are going on in the disease pool we all share in all the time. I suggest that you should be very, very frightened by these facts.