The devastating global impact of the acquired immunodeficiency syndrome (AIDS) has greatly sensitized the public to the threat of new microbes that are capable of wreaking havoc on the world's population. It is not surprising, therefore, that the unanticipated revelation last July at the Ninth International AIDS Conference in Amsterdam of a small number of people with markedly depressed CD4+ T-lymphocyte counts, with or without opportunistic diseases, in the absence of any evidence of human immunodeficiency virus (HIV) infection or other recognizable cause of immunosuppression resulted in extraordinary media attention and hence great public concern. Among the several subsequent meetings . . .