AIDS itself is not a sexually transmitted disease, but it is caused by one. HIV, the virus that leads to AIDS, is a sexually transmitted infection. AIDS is the late stage of untreated HIV infection, when the immune system has been severely damaged. The distinction matters because HIV is the infection you can catch and transmit, while AIDS is a condition that develops over time if HIV goes untreated.
HIV Is the STI, AIDS Is the Result
HIV (human immunodeficiency virus) is the virus that spreads between people. AIDS (acquired immunodeficiency syndrome) is what happens when HIV has destroyed enough of the immune system to leave the body vulnerable to serious infections and cancers. Think of it this way: HIV is the cause, and AIDS is the potential consequence years down the line.
A person receives an AIDS diagnosis when their CD4 cell count drops below 200 cells per cubic millimeter of blood. A healthy immune system typically has between 500 and 1,500 of these cells per cubic millimeter. CD4 cells are the white blood cells that coordinate your body’s defense against infections, so losing them leaves you open to illnesses a healthy immune system would normally fight off easily. An AIDS diagnosis can also be made when a person with HIV develops certain serious infections or cancers, regardless of their CD4 count.
These so-called AIDS-defining illnesses include a specific type of pneumonia caused by a fungus, Kaposi sarcoma (a cancer that causes lesions on the skin and organs), severe fungal infections of the throat or lungs, tuberculosis, toxoplasmosis of the brain, and chronic herpes infections lasting more than a month. There are over two dozen conditions on the list. None of them are unique to HIV, but their presence in someone with HIV signals that the immune system has failed significantly.
How HIV Spreads
Most people get HIV through anal or vaginal sex, or through sharing needles and syringes. The virus travels in specific body fluids: blood, semen, pre-seminal fluid, rectal fluids, vaginal fluids, and breast milk. For transmission to happen, these fluids need to contact a mucous membrane, damaged tissue, or enter the bloodstream directly through a needle.
During sex, the virus can enter through the lining of the rectum, the vagina and cervix, the opening at the tip of the penis, the foreskin, or any cuts or sores. Anal sex carries the highest risk of sexual transmission, particularly for the receptive partner, because the rectal lining is thin and easily damaged. Vaginal sex also transmits HIV in both directions. Oral sex carries a much lower risk.
HIV also passes from parent to child during pregnancy, childbirth, or breastfeeding. This is the most common way children acquire the virus. And sharing drug injection equipment remains a significant route because used needles can contain infected blood.
You cannot get HIV from hugging, shaking hands, sharing food or drinks, toilet seats, or insect bites. The virus does not survive long outside the body and is not spread through casual contact.
Why the Distinction Matters Today
In the 1980s and early 1990s, an HIV diagnosis almost inevitably progressed to AIDS. That is no longer the case. Modern antiretroviral treatment keeps the virus suppressed so effectively that many people with HIV will never develop AIDS. When someone on treatment maintains what’s called an undetectable viral load (meaning the amount of virus in their blood is too low to measure on standard tests), they have zero risk of transmitting HIV to sexual partners. The CDC confirms this principle, often summarized as U=U: undetectable equals untransmittable.
This is why calling AIDS an STD is misleading. AIDS is not something you “catch” from another person. You catch HIV, and AIDS only develops if the virus is left unchecked. With consistent treatment, a person diagnosed with HIV today can expect a near-normal lifespan and will likely never progress to AIDS.
The Global Picture
An estimated 40.8 million people were living with HIV worldwide at the end of 2024, including 1.4 million children. In that same year, 630,000 people died from HIV-related causes. Since the epidemic began, HIV has claimed roughly 44.1 million lives globally. The vast majority of deaths occur in regions with limited access to treatment, reinforcing how effective antiretroviral therapy is at preventing the progression from HIV to AIDS when it’s available.
Getting Tested
Because HIV often causes no symptoms for years, testing is the only reliable way to know your status. How soon a test can detect the virus after exposure depends on the type of test. A nucleic acid test, which looks for the virus itself in your blood, can detect HIV as early as 10 to 33 days after exposure. An antigen/antibody lab test using blood drawn from a vein works within 18 to 45 days. A rapid antigen/antibody test from a finger stick takes 18 to 90 days to become reliable. And antibody-only tests, including most home test kits, need 23 to 90 days.
If you think you’ve been exposed, testing too early can produce a false negative. The safest approach is to test at the earliest window for your test type and then retest after 90 days if the first result is negative. Many clinics, health departments, and pharmacies offer free or low-cost testing, and home test kits are available over the counter.