AIDS (acquired immunodeficiency syndrome) is the most severe stage of HIV infection, where the virus has damaged the immune system so extensively that the body can no longer fight off infections and certain cancers. It develops when a specific type of white blood cell, called a CD4 cell, drops below 200 cells per milliliter of blood. A healthy person typically has between 500 and 1,500. With modern treatment, most people with HIV never reach this stage.
How HIV Leads to AIDS
AIDS doesn’t appear on its own. It’s caused by HIV (human immunodeficiency virus), which specifically targets and destroys CD4 cells, the white blood cells responsible for coordinating your immune response against viruses, bacteria, and fungi. HIV hijacks these cells, inserting its own genetic material into the cell’s DNA and turning each infected cell into a factory for new virus particles. A single infected CD4 cell can produce roughly 10,000 new viral copies before it’s destroyed.
HIV infection moves through three stages. The first, acute infection, develops within two to four weeks of exposure. During this phase, the virus multiplies rapidly and the level of virus in the blood spikes, often causing flu-like symptoms. The second stage, chronic infection, can last a decade or longer. The virus is still active but replicates at much lower levels, and many people have no symptoms at all during this period. Without treatment, chronic infection eventually progresses to AIDS, the third and final stage, as the immune system loses too many CD4 cells to function.
What Happens to the Body During AIDS
Once the immune system is severely weakened, the body becomes vulnerable to infections it would normally handle with ease. These are called opportunistic infections because they take advantage of the damaged immune system. Some of the most common include a serious form of pneumonia caused by a fungus (Pneumocystis jirovecii), chronic intestinal infections, toxoplasmosis (a parasitic brain infection), tuberculosis, and severe fungal infections of the throat or lungs.
Certain cancers are also strongly associated with AIDS. Kaposi sarcoma, a cancer that causes lesions on the skin and internal organs, was one of the first conditions that drew attention to the epidemic in the early 1980s. Various types of lymphoma, including brain lymphoma, and invasive cervical cancer are also classified as AIDS-defining conditions. A person with HIV receives an AIDS diagnosis either when their CD4 count falls below 200 or when they develop any of these specific illnesses, regardless of their CD4 count.
Other serious complications include HIV-related encephalopathy (progressive brain damage), chronic herpes infections, severe weight loss known as wasting syndrome, and vision loss from viral eye infections.
How HIV Spreads
HIV transmits through specific body fluids: blood, semen, vaginal fluids, rectal fluids, and breast milk. Not all exposures carry equal risk. Blood transfusion with infected blood is by far the most efficient route, though modern blood screening has made this extremely rare in most countries. Mother-to-child transmission during pregnancy, birth, or breastfeeding carries the next highest risk per exposure.
For sexual transmission, receptive anal intercourse has the highest per-act risk, estimated 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 carries very low risk. Sharing needles for injection drug use falls in between, at roughly 63 per 10,000 exposures. Condom use combined with antiretroviral treatment of the HIV-positive partner reduces sexual transmission risk by over 99%.
Treatment Has Changed the Outlook Dramatically
Before effective treatment existed, an AIDS diagnosis was essentially a death sentence, with most people surviving only one to three years. That is no longer the case. Antiretroviral therapy (ART) works by blocking HIV at different points in its life cycle. Some drugs prevent the virus from entering CD4 cells. Others stop it from copying its genetic material, inserting its DNA into the cell, or assembling new viral particles. Modern treatment typically combines drugs from multiple classes into a single daily pill.
The results are striking. A 2023 analysis published in The Lancet, covering more than 200,000 people with HIV in Europe and North America, found that a 40-year-old man who started treatment after 2015 could expect to live to about 77. A 40-year-old woman could expect to live to roughly 79. These numbers approach, though don’t quite match, the general population’s life expectancy. The key factor is how early treatment begins: people who start ART while their CD4 counts are still at healthy levels (500 or above) gain significantly more life years than those who begin treatment late.
Even people who have already progressed to AIDS benefit substantially from treatment. A 40-year-old woman with a very low CD4 count (under 49 cells) who started treatment after 2015 could still expect about 25 additional years of life.
Undetectable Means Untransmittable
One of the most significant developments in HIV science is the confirmation that effective treatment also prevents transmission. When ART suppresses the virus to undetectable levels in the blood, a person with HIV cannot sexually transmit the virus to a partner. This principle, known as U=U (Undetectable = Untransmittable), is backed by multiple large clinical trials. The NIH-funded HPTN 052 trial found zero linked transmissions among couples where the HIV-positive partner maintained a suppressed viral load. The PARTNER and Opposites Attract studies confirmed the same finding for both heterosexual and male-male couples.
This requires taking ART consistently and maintaining an undetectable viral load through regular monitoring. But for those who do, the risk of sexual transmission is effectively zero.
Prevention for People Without HIV
Beyond treatment as prevention, two medical strategies exist for people who don’t have HIV. Pre-exposure prophylaxis (PrEP) involves taking medication before potential exposure, and when used consistently, it is highly effective at preventing infection. Post-exposure prophylaxis (PEP) is an emergency option taken within 72 hours of a possible exposure, reducing the risk of infection by more than 80%, likely much higher when taken exactly as prescribed.
The Global Picture
An estimated 40.8 million people worldwide were living with HIV at the end of 2024. Despite major progress in treatment access, about 630,000 people still died from HIV-related causes that year. The gap between those numbers reflects both how far treatment has come and how far access still needs to expand. Most AIDS-related deaths today occur not because effective treatment doesn’t exist, but because people are diagnosed too late or lack consistent access to medication.