皮肤恶性肿瘤 （卡波氏肉瘤， Kaposi sarcoma，简称 KS， 一种恶性度不是很高的皮肤血管肉瘤）是发现 AIDS 和 HIV 的导火索。本来这种肉瘤是主要发生于老年人的恶性肿瘤，表现为皮损和慢性病程，很少死亡。 1981年6月到8月，美国疾病控制中心陆续接到了96例男性同性恋者患卡波氏肉瘤和卡氏肺囊虫肺炎的报告。这些人年龄16-51岁，平均39岁，多数人伴有巨细胞病毒、单纯瘤彦病毒、隐球菌、弓形体等机会性感染，同时还有细胞免疫功能的缺陷。而在1970-1979年近10年的时间内没有发现一例50岁以下男性卡波氏肉瘤，1961-1979年近20年时间内纽约大学医院也只发 现了3例。
当时旧金山地区几个皮肤科诊所陆陆续续发现了好多例年轻男性发生 KS 的患者，医生都感觉非常奇怪。
当然后来发现 KS 是人类疱疹病毒八型 （HHV-8) 引起的，跟 HIV 病毒本身没有关系。但是跟 HIV 感染以后发现的细胞免疫缺陷有关系。
男同性恋中 HIV 感染发病率高的原因主要就是
肛门和直肠下端的黏膜比阴道黏膜更容易破损而产生艾滋病毒的传播。
男同性恋性行为中使用 Condom 的比例比异性性行为中使用的比例少很多，因为 Condom 最初的作用主要还是为了避孕，男同性恋性行为不存在这方面的顾虑。
还有就是男同性恋的性伴侣比异性恋的性伴侣变换频率高很多，他们的性行为也有很大的随机性。
research has shown that being on the receiving end of anal intercourse is equally risky whether you're a man or a woman. The risk was estimated at 1.4 percent per sex act with an infected person -- about 18 times more risky than male-to-female vaginal intercourse.
The study authors estimate that if receptive anal intercourse were only as risky as vaginal intercourse, HIV cases would fall by 80 percent to 98 percent among gay and bisexual men over five years. They also estimate that cases would fall by 29 percent to 51 percent if more gay and bisexual men had sex in long-term relationships instead of casual encounters.
看看这个几年前的美国疾病控制中心CDC 的统计学数据：
How does HIV affect gay and bisexual men?
In the United States, gay, bisexual, and other men who have sex with men (MSM) are the population most affected by HIV. According to the Centers for Disease Control and Prevention (CDC), about 67% of people diagnosed with HIV in 2015 in the United States were gay and bisexual men.
Among all gay and bisexual men, African Americans are the racial/ethnic group most affected by HIV. CDC reports that in 2015, more African-American gay and bisexual men were newly diagnosed with HIV than white or Hispanic/Latino gay and bisexual men.
