(1) AIDS is an excellent example of the relationship between behavior, environment, and disease. (2) This disease was first noted in male homosexuals. (3) One person, Gaetan Dugas, an airline steward from Canada, played a key role in its rapid transmission, for he or one of his sex partners had sex with forty of the first 248 AIDS cases reported in the United States. (4) The disease then hit another group whose lifestyle also encouraged its transmission—intravenous drug users who shared needles. (5) The third of the groups that were the hardest hit represents an environmental risk: hemophiliacs, who need regular blood transfusions, were exposed to the disease through contaminated blood. (6) Lifestyle was also central to how the disease entered the general population; the bridge was prostitutes who had sex with intravenous drug users and with bisexual and heterosexual men. (7) Lifestyle and environment continue to be significant: AIDS is more common among drug users who share needles and among people who have multiple sexual partners. The sentence that expresses the main idea is: