One underlying reason is that human activities have altered local, regional and global ecosystems[16]. We remain ignorant about many aspects of the ecology, epidemiology and natural history of some of the new diseases, but our understanding of others has grown with spectacular speed, a gratifying demonstration of the power of modern biomedical sciences, notably molecular biology and immunology. But the microbes are clever too, and some well-known ones have lately found new ways to challenge us, sometimes aided by features of modern civilization.