Emerging and re-emerging infectious diseases are epiphenomena of human existence and our interactions with each other and with nature. The growth of societies creates opportunities for infectious agents to emerge in new ecological niches. This is not new, except that our increasingly aggressive action on the environment induces increasingly extreme reactions from nature. Science will find medicines, vaccines and diagnostics that save lives. But to prevent new diseases we have to think collectively about how to live in harmony with nature, even though it can always surprise us. This chapter characterizes the scale of this problem and what can be done to prevent and control it.