The human immunodeficiency virus, or HIV, is a virus that attacks the immune system, specifically CD4 cells (or T cells).

The virus is transmitted through bodily fluids such as blood, semen, vaginal fluids, anal fluids, and breast milk. Historically, HIV has most often been spread through unprotected sex, the sharing of needles for drug use, and through birth.

Over time, HIV can destroy so many CD4 cells that the body can’t fight infections and diseases, eventually leading to the most severe form of an HIV infection: acquired immunodeficiency syndrome, or AIDS. A person with AIDS is very vulnerable to cancer and to life-threatening infections, such as pneumonia.

Though there is no cure for HIV or AIDS, a person with HIV who receives treatment early can live nearly as long as someone without the virus. And a study in 2019 in the medical journal, Lancet, showed that an anti-viral treatment effectively halted the spread of HIV.

Scientists have traced the origin of HIV back to chimpanzees and simian immunodeficiency virus (SIV), an HIV-like virus that attacks the immune system of monkeys and apes.

In 1999, researchers identified a strain of chimpanzee SIV called SIVcpz, which was nearly identical to HIV. Chimps, the scientist later discovered, hunt and eat two smaller species of monkeys—red-capped mangabeys and greater spot-nosed monkeys—that carry and infect the chimps with two strains of SIV. These two strains likely combined to form SIVcpz, which can spread between chimpanzees and humans.

SIVcpz likely jumped to humans when hunters in Africa ate infected chimps, or the chimps’ infected blood got into the cuts or wounds of hunters. Researchers believe the first transmission of SIV to HIV in humans that then led to the global pandemic occurred in 1920 in Kinshasa, the capital and largest city in the Democratic Republic of Congo.

The virus spread may have spread from Kinshasa along infrastructure routes (roads, railways, and rivers) via migrants and the sex trade.

In the 1960s, HIV spread from Africa to Haiti and the Caribbean when Haitian professionals in the colonial Democratic Republic of Congo returned home. The virus then moved from the Caribbean to New York City around 1970 and then to San Francisco later in the decade.

International travel from the United States helped the virus spread across the rest of the globe.

READ MORE: Pandemics That Changed History: A Timeline

Though HIV arrived in the United States around 1970, it didn’t come to the public’s attention until the early 1980s.

In 1981, the Centers for Disease Control and Prevention (CDC) published a report about five previously healthy homosexual men becoming infected with Pneumocystis pneumonia, which is caused by the normally harmless fungus Pneumocystis jirovecii. This type of pneumonia, the CDC noted, almost never affects people with uncompromised immune systems.

The following year, The New York Times published an alarming article about the new immune system disorder, which, by that time, had affected 335 people, killing 136 of them. Because the disease appeared to affect mostly homosexual men, officials initially called it gay-related immune deficiency, or GRID.

Though the CDC discovered all major routes of the disease’s transmission—as well as that female partners of AIDS-positive men could be infected—in 1983, the public considered AIDS a gay disease. It was even called the “gay plague” for many years after.

In September of 1982, the CDC used the term AIDS to describe the disease for the first time. By the end of the year, AIDS cases were also reported in a number of European countries.

READ MORE: Pandemics that Changed History

In 1984, researchers finally identified the cause of AIDS—the HIV virus—and the Food and Drug Administration (FDA) licensed the first commercial blood test for HIV in 1985.

Today, numerous tests can detect HIV, most of which work by detecting HIV antibodies. The tests can be done on blood, saliva, or urine, though the blood tests detect HIV sooner after exposure due to higher levels of antibodies.

In 1985, actor Rock Hudson became the first high-profile fatality from AIDS. In fear of HIV making it into blood banks, the FDA also enacted regulations that ban gay men from donating blood. The FDA would revise its rules in 2015 to allow gay men to give blood if they’ve been celibate for a year, though blood banks routinely test blood for HIV.

By the end of 1985, there were more than 20,000 reported cases of AIDS, with at least one case in every region of the world.

In 1987, the first antiretroviral medication for HIV, azidothymidine (AZT), became available.

Numerous other medications for HIV are now available, and are typically used together in what’s known as antiretroviral therapy (ART) or highly active antiretroviral treatment (HAART).

The regimes work by preventing the virus from multiplying, giving the immune system a chance to recover and fight off infections and HIV-related cancers. The therapy also helps reduce the risk of HIV transmission, including between an infected mother and her unborn child.

The World Health Organization (WHO), in 1988, declared December 1st to be World AIDS Day. By the end of the decade, there were at least 100,000 reported cases of AIDS in the United States and WHO estimated 400,000 AIDS cases worldwide.

In 1991, the red ribbon became an international symbol of AIDS awareness.

In that year, basketball player Magic Johnson announced he had HIV, helping to further bring awareness to the issue and dispel the stereotype of it being a gay disease. Soon after, Freddie Mercury—lead singer of the band Queen—announced he had AIDS and died a day later.

In 1994, the FDA approved the first oral (and non-blood) HIV test. Two years later, it approved the first home testing kit and the first urine test.

AIDS-related deaths and hospitalizations in developed countries began to decline sharply in 1995 thanks to new medications and the introduction of HAART. Still, by 1999, AIDS was the fourth biggest cause of death in the world and the leading cause of death in Africa.

WATCH: 30 Years of AIDs Research

In 2001, generic drug manufacturers began selling discounted copies of patented HIV drugs to developing countries, leading to several major pharmaceutical manufacturers slashing prices on their HIV drugs. The following year, the Joint United Nations Programme on HIV/AIDS (UNAIDS) reported that AIDS was by far the leading cause of death in sub-Saharan Africa.

In 2009, President Barack Obama lifted a 1987 U.S. ban that prevented HIV-positive people from entering the country.

The FDA approved pre-exposure prophylaxis, or PrEP, for HIV-negative people in 2012. When taken daily, PrEP can reduce the risk of HIV from sex by more than 90 percent and from intravenous drug use by 70 percent, according to the CDC. A major study completed in 2019 showed that over 750 gay men on an anti-viral treatment did not transmit the virus to their partners. "Our findings provide conclusive evidence that the risk of HIV transmission through anal sex when HIV viral load is suppressed is effectively zero," the paper, published in Lancet, stated.

At the end of 2019, some 38 million people were living with HIV/AIDS worldwide, and 940,000 people died from AIDS-related illnesses that year, according to WHO. Sub-Saharan Africa remains the most severely affected region, accounting for nearly two-thirds of the world’s current HIV cases.

Origin of HIV and AIDS: AVERT.
HIV Originated With Monkeys, Not Chimps, Study Finds: National Geographic.
HIV pandemic originated in Kinshasa in the 1920s, say scientists: The Guardian.
America’s HIV outbreak started in this city, 10 years before anyone noticed: PBS.
HIV Testing: CDC.
About HIV/AIDS: CDC.
How HIV spread across the West: CNN.
Obama Lifts a Ban on Entry Into U.S. by H.I.V.-Positive People: The New York Times.
Global Health Observatory (GHO) data: World Health Organization.