AIDS is caused by the human immunodeficiency virus, known as HIV. Specifically, there are two types: HIV-1 and HIV-2. HIV-1 is responsible for the vast majority of infections worldwide, while HIV-2 occurs predominantly in West Africa and produces lower levels of circulating virus in the body.
How HIV Attacks the Immune System
HIV targets a specific type of white blood cell called a CD4 cell. These cells coordinate your immune response, essentially directing other immune cells to fight off infections. HIV latches onto receptors on the surface of a CD4 cell, fuses with it, and slips inside.
Once inside, the virus converts its own genetic material into DNA and inserts it directly into the cell’s DNA. The infected cell then becomes a factory for new virus particles. During the acute stage of infection, a single CD4 cell can produce roughly 10,000 new HIV particles before being destroyed. Those particles go on to infect more CD4 cells, and the cycle repeats. Over time, this relentless process depletes the immune system’s ability to fight off even minor infections.
How HIV Spreads
HIV is transmitted through specific body fluids: blood, semen, rectal fluids, vaginal fluids, and breast milk. The risk varies enormously depending on the type of exposure. Blood transfusion with contaminated blood carries the highest risk at about 9,250 transmissions per 10,000 exposures. Sharing needles during injection drug use carries a risk of about 63 per 10,000 exposures.
For sexual transmission, receptive anal intercourse has the highest per-act risk at about 138 per 10,000 exposures. Receptive vaginal intercourse carries a risk of about 8 per 10,000 exposures, and insertive vaginal intercourse about 4 per 10,000. Oral sex, biting, spitting, and sharing sex toys all carry negligible or very low risk. These numbers represent single-exposure events, but risk accumulates with repeated exposures over time.
From HIV to AIDS
HIV infection and AIDS are not the same thing. HIV is the virus; AIDS is the most advanced stage of the disease it causes. A healthy person typically has a CD4 count between 500 and 1,500 cells per cubic millimeter of blood. AIDS is diagnosed when that count drops to 200 or below, or when a person develops certain serious infections or cancers that a healthy immune system would normally prevent.
Without treatment, chronic HIV infection usually advances to AIDS in about 10 years, though some people progress faster. During those years, the virus steadily chips away at CD4 cells. Early on, many people feel fine and may not know they’re infected. By the time AIDS develops, the immune system is so weakened that the body becomes vulnerable to a range of illnesses it would normally handle easily.
What Happens When AIDS Develops
The infections that strike people with AIDS are called opportunistic infections because they take advantage of a crippled immune system. These include a type of pneumonia caused by a fungus (Pneumocystis jirovecii), a parasitic brain infection called toxoplasmosis, chronic intestinal infections, and severe fungal infections of the throat and lungs. Certain cancers also emerge more frequently, including Kaposi sarcoma (a cancer that causes lesions on the skin and internal organs), invasive cervical cancer, and several types of lymphoma. Severe, prolonged herpes outbreaks and a wasting syndrome involving significant weight loss and chronic diarrhea are also hallmarks of AIDS.
Any of these conditions in a person with HIV can lead to an AIDS diagnosis, even if their CD4 count hasn’t dropped below 200.
Treatment and Life Expectancy Today
Modern antiretroviral therapy has transformed HIV from a death sentence into a manageable chronic condition. The medications work by interrupting the virus at different stages of its life cycle, preventing it from replicating and giving the immune system a chance to recover. People who start treatment early and take it consistently can maintain near-normal CD4 counts and reduce the amount of virus in their blood to undetectable levels.
A large collaborative study published in The Lancet found that a 40-year-old man starting treatment after 2015 could expect to live an additional 37 years on average, compared to about 41 years for a man in the general population. For women, the figures were 39 additional years with treatment versus nearly 46 in the general population. When CD4 counts remain high on treatment, the gap narrows to just a few years regardless of when treatment was started.
The global picture, however, remains serious. About 40.8 million people worldwide were living with HIV in 2024, and an estimated 630,000 people died from AIDS-related illnesses that year. The gap between outcomes in well-resourced healthcare systems and those in harder-hit regions remains one of the biggest challenges in the global HIV response.