Great epidemics of the past occurred following a collision between a rampant microbe and a totally defenceless population living in circumstances in which the disease could spread freely. The enormous menace of HIV and the prion diseases originate in the novelty of their disease-causing capability and their ability to exploit weaknesses in our hygienic arrangements. Other formerly dangerous diseases still have an undiminished ability to kill humans on a large scale. Malaria – less menacing outside of Africa than in former times – remains one of the world's greatest health problems. Tuberculosis is, in principle, curable, but the difficulty of implementing effective treatment means that it persists. Indeed, it infects one-third of the world's population, and in HIV infected people it is now creating a severe danger. The influenza virus has a history of creating unpleasant surprises, but it is now under control because of a remarkable surveillance and vaccination program. Bioterrorism makes health officials nervous. Complacent in our belief that smallpox has been eradicated, and with our immunity to smallpox steadily diminishing, we could be vulnerable to bioterrorists. The important lesson of the last thirty years is that we should never assume that microbial enemies are vanquished and should recognise that microbes exploit chinks in our hygienic armour.