A biological terrorist attack probably would first be detected by doctors or other health-care workers. The speed of a response would then depend on their rapid recognition and communication that certain illnesses appeared out of the ordinary. For this reason, preparing for biological terrorism has more in common with confronting the threat of emerging infectious diseases than with preparing for chemical or nuclear attacks. Defence against bioterrorism, like protection against emerging diseases, must therefore rely on improved national and international public-health surveillance. Too often, thinking about bioterrorism has mimicked thinking about chemical terrorism, a confusion that leads to an emphasis on the wrong approaches in preparing to meet the threat.