In human history, numerous infectious diseases have emerged and re-emerged. Aside from many others, the so-called 'exotic' agents in particular are a threat to our public health systems due to limited experience in case management and lack of appropriate resources. Many of these agents are zoonotic in origin and transmitted from animals to man either directly or via vectors. The reservoirs are often infected subclinically or asymptomatically and the distribution of the diseases basically reflects the range and the population dynamics of their reservoir hosts. As examples, emergence/re-emergence is discussed here for diseases caused by filoviruses, hantaviruses, paramyxoviruses, flaviviruses and Yersinia pestis. In addition, bioterrorism is addressed as one factor which has now to be considered in infectious disease emergence/re-emergence. Preparedness for known and unknown infectious diseases will be a top priority for our public health systems in the beginning of the millennium.