Abstract
Humankind has been besieged through-out its evolution by microorganisms that pose a continual challenge to the survival of the species.1 Although such ancient killers as tuberculosis and malaria persistently take a toll of millions of lives per year, occasionally the emergence or reemergence of a microbe results in an unexpected, catastrophic pandemic with global public health consequences. As we prepare to leave the 20th century, it is worth reflecting on the fact that within the framework of an enormous but constant burden of a variety of infectious diseases, as well as a number of mini-epidemics, this century has witnessed two such unexpected cataclysmic events.