In July 1976 a pneumonialike epidemic from a previously unrecognized microorganism erupted among Legionnaires who had attended a meeting in Philadelphia. There were an estimated 182 cases, in which 29 patients died. Among other things the episode shows that even in a medically sophisticated industrialized nation, a bacterial pathogen can produce a small epidemic and defy identification for almost 6 months. One historical implication of the event is the need to consider the possibility of a return of large-scale epidemic disease rivaling the sweeps of bubonic plague in fourteenth-century Europe. Such epidemics could occur through any of a variety of microorganismic mechanisms recognized as operating at the present time. It is suggested that humans would react to such a disaster much as their progenitors did centuries ago.