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The widespread occurrence of KS and PCP in young people with no underlying disease or history of immunosuppressive therapy was unprecedented. Searches of the medical literature, autopsy records and tumor registries revealed that these diseases previously had occurred at very low levels in the United States (CDC, 1981b; CDC, 1982f).

KS, a very rare skin neoplasm, had affected mostly older men of Mediterranean origin or cancer or transplant patients undergoing immunosuppressive therapy (Gange and Jones, 1978; Safai and Good, 1981). Before the AIDS epidemic, the annual incidence of Kaposi's sarcoma in the United States was 0.02 to 0.06 per 100,000 population (Rothman, 1962a; Oettle, 1962). In addition, a more aggressive form of KS that generally occurred in younger individuals was seen in certain parts of Africa (Rothman, 1962b; Safai, 1984a). By 1984, never-married men in San Francisco were found to be 2,000 times more likely to develop KS than during the years 1973 to 1979 (Williams et al., 1994). As of Dec. 31, 1994, 36,693 patients with AIDS in the United States with a definitive diagnosis of KS had been reported to the CDC (CDC, 1995b).

PCP, a lung infection caused by a pathogen to which most individuals are exposed with no undue consequences, was extremely rare prior to 1981 in individuals other than those receiving immunosuppressive therapy or among the chronically malnourished, such as certain Eastern European children following World War II (Walzer, 1990). A 1967 survey, for example, found only 107 U.S. cases of PCP reported in the medical literature up to that point, virtually all among individuals with underlying immunosuppressive conditions or who had undergone immunosuppressive therapy (Le Clair, 1969). In that year, CDC became the sole supplier in the United States of pentamidine isethionate, then the only recommended PCP therapy, and began collecting data on each PCP case diagnosed and treated in this country. After reviewing requests for pentamidine in the period 1967 to 1970, researchers found only one case of confirmed PCP without a known underlying condition (Walzer et al., 1974). In the period immediately prior to the recognition of AIDS, January 1976 to June 1980, CDC received only one request for pentamidine isethionate to treat an adult in the United States who had PCP and no underlying disease (CDC, 1982f). In 1981 alone, 42 requests for pentamidine were received to treat patients with PCP and no known underlying disorders (CDC, 1982f). By Dec. 31, 1994, 127,626 individuals with AIDS in the United States with definitive diagnoses of PCP had been reported to the CDC (CDC, 1995b).

Another rare opportunistic disease, disseminated infection with the Mycobacterium avium complex (MAC), also was seen frequently in the first AIDS patients (Zakowski et al., 1982; Greene et al., 1982). Prior to 1981, only 32 individuals with disseminated MAC disease had been described in the medical literature (Masur, 1982a). By Dec. 31, 1994, the CDC had received reports of 28,954 U.S. AIDS patients with definitive diagnoses of disseminated MAC (CDC, 1995b).