HIV is caused by a virus, specifically the human immunodeficiency virus, which belongs to a family of viruses called retroviruses. Unlike most viruses, retroviruses carry their genetic instructions as RNA and convert them into DNA inside human cells, essentially hijacking the cell’s own machinery to make copies of themselves. There are two types: HIV-1, which causes the vast majority of infections worldwide, and HIV-2, which is found mostly in West Africa and progresses more slowly.
How HIV Infects the Body
HIV targets a specific type of white blood cell called a CD4 cell. These cells act as coordinators of your immune response, helping your body fight off infections from bacteria, viruses, and fungi. The virus latches onto receptors on the surface of a CD4 cell, fuses with its outer membrane, and slips inside.
Once inside, HIV converts its RNA into DNA using a specialized enzyme, then inserts that DNA directly into the cell’s own genetic code. At that point, the infected cell begins producing new copies of the virus. Those copies break free, destroy the host cell, and move on to infect more CD4 cells. Over time, this cycle steadily depletes the immune system. A healthy person typically has between 500 and 1,500 CD4 cells per cubic millimeter of blood. When the count drops to 200 or below, the infection has progressed to AIDS, the stage where the body can no longer defend itself against infections it would normally handle easily.
How HIV Spreads Between People
HIV can only spread through specific body fluids: blood, semen (including pre-seminal fluid), vaginal fluids, rectal fluids, and breast milk. For transmission to occur, one of these fluids must come into contact with a mucous membrane, damaged tissue, or enter the bloodstream directly through a needle.
The most common route is unprotected vaginal or anal sex. The risk varies significantly depending on the type of exposure. Per 10,000 exposures with an HIV-positive partner (without condoms or medication), the estimated transmission risk breaks down like this:
- Receptive anal sex: about 1 in 72
- Insertive anal sex: about 1 in 909
- Receptive vaginal sex: about 1 in 1,250
- Insertive vaginal sex: about 1 in 2,500
- Oral sex: rare but not impossible
Sharing needles or drug injection equipment is another significant route, with an estimated risk of 63 per 10,000 exposures. This applies not just to injection drug use but also to shared equipment for tattoos or piercings that hasn’t been properly sterilized. A mother can also pass HIV to her baby during pregnancy, childbirth, or breastfeeding, though antiretroviral treatment during and after pregnancy dramatically reduces this risk.
HIV does not spread through casual contact. Hugging, shaking hands, sharing food, using the same toilet, or being around someone who coughs or sneezes cannot transmit the virus. Saliva, sweat, and tears do not carry enough virus to cause infection.
HIV-1 vs. HIV-2
HIV-1 is responsible for the global pandemic and accounts for over 99% of infections in the United States. HIV-2, concentrated in West Africa and countries with historical ties to the region (particularly France, Spain, Portugal, and former Portuguese colonies like Brazil and parts of India), behaves quite differently. It produces lower levels of virus in the blood, leads to a slower decline in immune function, and transmits less efficiently through both sexual contact and childbirth. In studies conducted before antiretroviral treatment was available, the rate of mother-to-child transmission for HIV-2 was under 5%, compared to roughly 25% for HIV-1.
That said, HIV-2 is not harmless. Research from West Africa shows that most people with untreated HIV-2 will eventually develop AIDS within about 15 years. It simply takes longer to get there.
Where the Virus Originally Came From
HIV did not appear out of nowhere. Both types crossed into humans from primates in Africa. HIV-1 originated from a closely related virus found in central African chimpanzees, with at least three separate crossover events giving rise to different groups of the virus. The most widespread of these, HIV-1 group M (the one responsible for the global epidemic), likely entered the human population around 1908, based on genetic analysis of the virus’s evolutionary history. Gorillas may have served as an intermediate host for one of the other groups.
HIV-2 came from a different primate entirely: the sooty mangabey, a monkey native to West Africa. At least eight separate transmission events from mangabeys to humans produced the various HIV-2 groups, with the two main ones dating to the early 1930s. In both cases, the crossover almost certainly happened through hunting and butchering of primates, which would have exposed people to infected blood.
Undetectable Means Untransmittable
One of the most important developments in HIV science is the confirmation that a person on effective treatment who maintains an undetectable viral load has zero risk of sexually transmitting HIV. This concept, known as U=U (undetectable equals untransmittable), is backed by large studies tracking thousands of couples where one partner had HIV and the other did not. Not a single linked transmission occurred when the HIV-positive partner had a suppressed viral load.
Antiretroviral treatment works by blocking the virus at various stages of its life cycle, preventing it from making new copies and reducing the amount of virus in the blood to levels too low to measure on standard tests. This doesn’t cure the infection (the virus’s DNA remains embedded in some cells), but it keeps the immune system intact and eliminates the risk of passing HIV to sexual partners.