This paper discusses the ways in which the sharply increased danger of bio-terrorism has made infectious diseases a priority in defence and intelligence circles. Against this background, the author sets out a central principle of global public health security: a strengthened capacity to detect and contain naturally caused outbreaks is the only rational way to defend the world against the threat of a bio-terrorist attack. He then discusses the three trends that underscore this point: vulnerability of all nations to epidemics, the capacity of a disease such as AIDS to undermine government and society, and the way in which the determinants of national security have been re-defined in the post-Cold War era.