Young and Meyer (2005) note that the term has been in use within the public health discourse since 1990 or earlier, but that the coining of the initialism by Glick et al. (1994) "signaled the crystallization of a new concept. They trace the emergence of this behavioural concept to two very distinct academic perspectives. First, it was pursued by epidemiologists seeking behavioral categories that would offer better analytical concepts for the study of disease risk than identity-based categories (such as "gay", "lesbian", or "straight", because a man who self-identifies as gay or bisexual is not necessarily sexually active with men). Second, its usage could in part be explained by the criticism of sexual identity terms prevalent in the "social construction" literature, which typically rejected the use of identity-based concepts across different cultural and historical contexts.
As a risk category, MSM are not limited to small self-identified and visible sub-populations, such as gay men and male sex workers. MSM and homosexual refer to different things: behaviors and social identities. MSM refers to sexual relationships between men, whether or not they identify as homosexual or bisexual. Homosexual orientation refers to sexual/romantic attraction between men and does not necessarily imply any history of sexual relationships. Homosexuality refers to more than the sexual relationship and may extend to broader relationships with the same sex. Its precise use and definition has varied with regard to transwomen. Some sources consider tranwomen to be MSM, others considering transwomen "along side" MSM, and others are internally inconsistent (defining transgender women to be MSM in one place but referring to "MSM and transgender" in another).
In their assessment of the knowledge about the sexual networks and behaviors of men who have sex with men in Asia, Dowsett, Grierson and McNally concluded that the category of MSM does not correspond to a single social identity in any of the countries they studied.
"The literature reveals that there are no socially or self-defined groups of men that fit into an overarching category of MSM. What the review shows is that there are just men!! Fishermen, students, factory workers, military recruits, truck drivers, and men who sell sex, and so on: all these categories of men are to be found in the studies and programmes reviewed."
There were no similar traits in all of the MSM population studied, other than them being males and engaging in sex with other men.
Infection with the Hepatitis B virus is about 5-6 times more common, and Hepatitis C virus infections are about 2 times more common, in men who have sex with men than in the general population. They also have an increased incidence and prevalence of Kaposi's sarcoma-associated herpesvirus, which causes a cancer called Kaposi's sarcoma in immunocompromised individuals. In 2006, 64% of the reported syphilis cases in the United States were among men who have sex with men. Men who have sex with men are 17 times more likely to develop anal cancer than heterosexual men, probably due to a higher risk of human papillomavirus. Many people become infected with HPV soon after becoming sexually active. MSM also have high rates of intestinal parasitism throughout the world.
A comparison study of HIV-infected men found that those who had sex with men were especially unlikely to receive HIV preventative services even though they were more likely to report unprotected sexual intercourse with seronegative and unknown serostatus casual partners. This can lead to the rapid transmission of HIV among small clusters of gay men.
A 1990 study called The Social Organization of Sexuality showed that men who had at least one male sexual partner in the previous 5 years had an average of 16.7 sexual partners during that time period, while men who only had sex with women had an average of 4.8 sexual partners during that time.
One study based on the Amsterdam Cohort Study (which includes only MSM who have had two sexual partners in the previous six months) concluded that young MSM were more likely to have contracted HIV from a steady partner than from a casual partner, compared with older MSM, possibly due to higher rates of unprotected anal intercourse.
However, the persistence of disparities in HIV between heterosexual individuals and MSM in the United States cannot be explained solely by differences in risky sexual behavior between these two populations; it is also contributed to by both the lack of "sexual role segregation" between male sex partners and "the differential anal/vaginal transmission probabilities".
James Chin, clinical professor of epidemiology at UC Berkeley, has charged that political correctness has led to the undertargeting of those at greatest risk of HIV, including MSM, in HIV/AIDS prevention programmes.
In developing countries, homosexual relationships may be illegal, making MSM difficult to reach. Studies have found that less than 5 percent of MSM in Africa, Asia, and Latin America have access to HIV-related health care.