Since reports of emergence and spread of the human immunodeficiency virus (HIV) in the United States between the 1970s and 1980s, the HIV/AIDS epidemic has frequently been linked to gay, bisexual, and other men who have sex with men (MSM) by epidemiologists and medical professionals. It was first noticed after doctors discovered clusters of Kaposi's sarcoma and pneumocystis pneumonia in homosexual men in Los Angeles, New York City, and San Francisco in 1981. The first official report on the virus was published by the Centers for Disease Control (CDC) on June 5, 1981, and detailed the cases of five young gay men who were hospitalized with serious infections. A month later, The New York Times reported that 41 homosexuals had been diagnosed with Kaposi's sarcoma, and eight had died less than 24 months after the diagnosis was made.

By 1982, the condition was referred to in the medical community as "gay-related immune deficiency" (GRID), "gay cancer", and "gay compromise syndrome".Oswald, G.A, et al (1982) 'Attempted immune stimulation in the "gay compromise syndrome"'. BMJ, 1982 October 16; 285(6348): 1082. It was not until July 1982 that the term Acquired Immune Deficiency Syndrome (AIDS) was suggested to replace GRID,Grmek, M.D. (1990) 'History of AIDS: Emergence and origin of a modern pandemic', New Jersey: Princeton University Press. and it was not until September that the CDC first officially used the AIDS acronym.MMWR Weekly (1982) 'Current Trends Update on Acquired Immune Deficiency Syndrome (AIDS) – United States', September 24, 31(37); 507–508, 513-514. Scientists and physicians now know that HIV/AIDS does not only affect MSM and can infect anybody, regardless of sex and sexual orientation. Nonetheless, MSM are still considered a "key population" globally, meaning they have high rates of HIV and are at high risk for acquiring it.

Gay, bisexual, and other men who have sex with men are a small percentage of the U.S. population, but are consistently the population group most affected by the HIV/AIDS epidemic in the United States, and are the largest proportion of American citizens with an AIDS diagnosis who have died.Centers for Disease Control. "HIV among gay, bisexual, and other men who have sex with men (MSM)" (2010). Department of Health and Human Services. The United Nations estimates the global median HIV prevalence among MSM at 7.7%.

MSM as a behavioral category

Men who have sex with men (abbreviated as MSM, also known as males who have sex with males) are males who engage in sexual activities with members of the same sex, regardless of how they personally identify themselves. Many MSM choose not to (or cannot for other reasons) identify as homosexual or bisexual. Similarly, the label excludes men who identify as gay or bisexual, but who have never had sex with another man, including many gay teenagers.

The terms "men who have sex with men" (MSM) and "women who have sex with women" (WSW) have been used in medical scholarship and research since at least 1990.Young, Rebecca M. and Ilan H. Meyer. "The Trouble With "MSM" and "WSW": Erasure of the Sexual-Minority Person in Public Health Discourse." Am J Public Health. (2005) 95: 1144–1149. But, the term has been attributed to Glick et al., because their usage in a 1994 study solidified the concept in medical terminology. MSM is often used in medical literature and social research to describe such men as a group for research studies without considering issues of self-identification because it offers better behavioral categories for the study of disease-risk than identity-based categories (such as "gay", "bisexual", or "straight"), because a man who self-identifies as gay or bisexual is not necessarily sexually active with men, and someone who identifies as straight might be sexually active with men.

Demographics Determining the number of men who have ever had sex with another man is difficult worldwide. The World Health Organization estimates that at least 3% and as high as 16% of men have had sex at least once with a man. Their estimate includes victims of sexual abuse in addition to men who regularly or voluntarily have sex with men. The United Nations estimates that 6-20% of men worldwide have sex with other men at some point during their lifetime."Men who have sex with men, HIV prevention and care" Geneva, November 2005. UNAIDS. http://data.unaids.org/pub/Report/2006/jc1233-msm-meetingreport_en.pdf A recent study using social media platforms to estimate the global population of MSM resulted in much higher estimates than those reported by the UNAIDS.

MSM in the United States

Estimates about the U.S. population of men who have sex with men (MSM) vary. The Centers for Disease Control and Prevention estimate that men who have sex with men (MSM) represent about 2% of the American population. A 2005 study estimates that among U.S. men aged 15 to 44, an estimated 6% of have engaged in oral and/or anal sex with another man at some point in their lives, and about 2.9% have had at least one male partner in the previous 12 months. A 2007 study estimated that there are 7.1 million men who have sex with men (MSM) in the United States, or 6.4% of the overall U.S. population. The percentage of men who have sex with men varied by state, with the lowest percentage reported in South Dakota (3.3%) and the highest in the District of Columbia (13.2%). However, the same study found that 57% of men who have sex with men identified as heterosexual or bisexual.Lieb, S., Fallon, S., Friedman, S., Thompson, D., Gates, G., Liberti, T., & Malow, R. (2011). Statewide estimation of racial/ethnic populations of men who have sex with men in the U.S. PubMed, 126(1), 60-72. A 2010 study estimated that 2.6% of the male population in the United States had engaged in same-sex behavior in the past year, 4.0% in the past five years, and 7.0% at any point in their lifetime.Purcell, D.W., C Johnson, A Lansky, J Prejean, R Stein, P Denning, Z Gaul, H Weinstock, J Su, & N Crepaz. "Calculating HIV and Syphilis Rates for Risk Groups: Estimating the National Population Size of Men Who Have Sex with Men" Latebreaker #22896 Presented March 10, 2010. 2010 National STD Prevention Conference; Atlanta, GA.

"Down-low" subculture among Black MSM

Down-low is an African-American slang term specifically used within the African-American community that typically refers to a sexual subculture of Black men who usually identify as heterosexual but actively seek sexual encounters and relations with other men, practice gay cruising, and frequently adopt a specific hip-hop attire during these activities. They generally avoid disclosing their same-sex sexual activities, and often have female sexual partner(s), and may be married or single.

In medical research, the term down-low is used to identify sexual identity-behaviour discordance among men who have sex with men (MSM). According to a study published in the Journal of Bisexuality, "[t]he Down Low is a lifestyle predominately practiced by young, urban Black men who have sex with other men and women, yet do not identify as gay or bisexual".

In this context, "being on the down-low" is more than just men having sex with men in secret, or a variant of closeted homosexuality or bisexuality—it is a sexual identity that is, at least partly, defined by its "cult of masculinity" and its rejection of what is perceived as White American culture (including what is perceived as White American LGBT culture) and terms. A 2003 cover story in The New York Times Magazine on the down-low phenomenon explains that the American Black community sees "homosexuality as a white man's perversion." It then goes on to describe the down-low subculture as follows:

The CDC cited three findings that relate to African-American men who operate on the down-low (engage in MSM activity but do not disclose to others): African American men who have sex with men (MSM), but who do not disclose their sexual orientation (nondisclosers), have a high prevalence of HIV infection (14%); nearly three times higher than nondisclosing MSMs of other races/ethnicities (5%). Confirming previous research, the study of 5,589 MSM, aged 15–29 years, in six U.S. cities found that African American MSM were more likely not to disclose their sexual orientation compared with white MSM (18% vs. 8%). HIV-infected nondisclosers were less likely to know their HIV status (98% were unaware of their infection compared with 75% of HIV-positive disclosers), and more likely to have had recent female sex partners.

Risk factors According to UNAIDS, in 2018, MSM globally have 22 times higher risk of acquiring HIV compared to all adult men.

A 2007 study analyzing two large population surveys found that "the majority of gay men had similar numbers of unprotected sexual partners annually as straight men and women."Sexual Behavior Does Not Explain Varying HIV Rates Among Gay And Straight Men However, a 2006 study found that men who reported 4 or more male sexual partners were at increased risk of HIV infection. Study participants who reported amphetamine or heavy alcohol use before sex were more likely to have HIV or other sexually transmitted infections.BA Koblin. "Risk factors for HIV infection among men who have sex with men" (2006) AIDS, 20. 731-739.

A Kaiser Family Foundation study indicated that fewer Americans view HIV as a top health priority today compared to ten years ago. In 1996, 25% of Americans viewed HIV as an "urgent problem" to their community but in 2009, only 17% listed it as "urgent." The percentage of 18- to 29-year-olds that were personally concerned about contracting the virus dropped from 28% in 1995 to 17% in 2009 Kaiser Family Foundation. (2009) "2009 Survey of Americans on HIV/AIDS: summary of findings on the domestic epidemic." A study conducted in 6 major U.S. cities found that only one in 4 teenage men who have sex with men believed they were personally at risk for contracting the HIV virus.MacKellar DA, Valleroy LA, Secura GM, et al. (2007) Perceptions of lifetime risk and actual risk for acquiring HIV among young men who have sex with men. AIDS Behav:263-270.

Unprotected anal intercourse