AIDS was first clinically observed in 1981 in the United States.[23] The initial cases were a cluster of injecting drug users and homosexual men with no known cause of impaired immunity who showed symptoms of Pneumocystis carinii pneumonia (PCP), In the United States, as of 2009, most sexual transmission occurred in men who had sex with men,[2] with this population accounting for 64% of all new cases. 来自 wikipedia.