HIV (human immunodeficiency virus) is a virus that attacks the immune system, specifically the cells your body relies on to fight infections. AIDS (acquired immunodeficiency syndrome) is the most advanced stage of HIV infection, diagnosed when the immune system has been severely damaged. They are not the same thing: HIV is the virus, and AIDS is a condition that can develop years later if HIV goes untreated. With modern treatment, most people with HIV never develop AIDS.
How HIV Attacks the Immune System
HIV targets a specific type of white blood cell called a CD4 cell. These cells act as coordinators of your immune response, signaling other immune cells to fight off bacteria, viruses, and other threats. HIV latches onto a CD4 cell, fuses with it, and slips inside. Once there, it hijacks the cell’s own machinery to make copies of itself. A single infected CD4 cell can produce roughly 10,000 new viral particles. Those new copies burst out of the cell, destroying it in the process, and go on to infect more CD4 cells.
Over time, this cycle kills off enough CD4 cells that the immune system can no longer mount an effective defense. A healthy person typically has between 500 and 1,500 CD4 cells per cubic millimeter of blood. When that number drops below 200, the body becomes vulnerable to infections it would normally handle with ease.
The Three Stages of HIV Infection
Stage 1: Acute Infection
Within two to four weeks of contracting HIV, many people experience flu-like symptoms: fever, headache, rash, and body aches. This is when the virus is multiplying rapidly and the viral load in the blood is extremely high, making transmission more likely. Some people have no symptoms at all during this stage.
Stage 2: Chronic (Latent) Infection
After the initial burst, HIV continues replicating at much lower levels. This stage can last a decade or longer without treatment, and many people feel perfectly fine during it. The virus is still active and still damaging the immune system, just slowly. With treatment, people can remain in this stage for several decades.
Stage 3: AIDS
AIDS is diagnosed when the CD4 count falls below 200 cells per cubic millimeter, or when a person develops specific serious infections that only take hold when the immune system is severely weakened. These are called opportunistic infections and include certain types of pneumonia, tuberculosis, and specific cancers. Without treatment, people with AIDS typically survive about three years.
How HIV Spreads
HIV can only be transmitted through specific body fluids: blood, semen, pre-seminal fluid, rectal fluids, vaginal fluids, and breast milk. These fluids must come into contact with a mucous membrane (found inside the rectum, vagina, penis, and mouth), damaged tissue, or be injected directly into the bloodstream.
The most common routes of transmission are anal sex, vaginal sex, and sharing needles or syringes. Receptive anal sex carries the highest risk among sexual activities. HIV can also pass from mother to child during pregnancy, childbirth, or breastfeeding.
Oral sex carries very low risk. Deep open-mouth kissing has only been documented as a transmission route in rare cases where both people had open sores or bleeding gums. You cannot get HIV from saliva, casual contact, sharing food or drinks, or closed-mouth kissing.
Testing and Window Periods
No HIV test can detect the virus immediately after exposure. The gap between infection and when a test can pick it up is called the window period, and it varies by test type.
- Nucleic acid test (NAT): Detects the virus itself in the blood. Window period of 10 to 33 days. This is the earliest possible detection method.
- Antigen/antibody test (blood draw): Looks for both a viral protein called p24 and the antibodies your body produces in response to HIV. Window period of 18 to 45 days from a vein draw, or 18 to 90 days from a finger stick.
- Antibody test: Looks only for antibodies. Window period of 23 to 90 days. This category includes rapid tests and the only FDA-approved home self-test.
A negative result is only reliable if enough time has passed since the most recent potential exposure. If you test too early, you may need to test again.
Treatment and Life Expectancy
HIV is treated with antiretroviral therapy, commonly called ART. ART is a combination of medications that block the virus at different points in its replication process. Some prevent the virus from entering CD4 cells. Others sabotage the enzymes HIV uses to copy its genetic material or assemble new viral particles. Taken together, these drugs reduce the amount of virus in the body to levels so low that standard tests cannot detect it, typically fewer than 20 copies per milliliter of blood.
This matters enormously for two reasons. First, keeping viral levels undetectable allows the immune system to recover and stay functional. Second, a person with an undetectable viral load has zero risk of transmitting HIV to sexual partners. The CDC has confirmed this finding, often summarized as “Undetectable = Untransmittable” or U=U.
Life expectancy for people on ART has improved dramatically. A 40-year-old man who started treatment after 2015 can expect to live to about 77, compared to roughly 81 in the general population, according to a large collaborative study published in The Lancet HIV. For those who maintain high CD4 counts on treatment, life expectancy is only a few years shorter than average. This is a striking contrast to the era before effective treatment, when an AIDS diagnosis was almost universally fatal within a few years.
Prevention
Beyond condoms, which remain highly effective when used consistently, there are now pharmaceutical tools to prevent HIV infection. Pre-exposure prophylaxis (PrEP) is medication taken by HIV-negative people before potential exposure. When taken as prescribed, PrEP reduces the risk of getting HIV from sex by about 99%. Among people who inject drugs, it reduces risk by at least 74%.
Post-exposure prophylaxis (PEP) is an emergency option taken after a potential exposure. It must be started within 72 hours to be effective and involves taking medication for 28 days.
The U=U principle also functions as prevention: when someone living with HIV maintains an undetectable viral load through treatment, they cannot pass the virus to sexual partners. This has fundamentally changed how public health experts think about stopping HIV transmission.
HIV Today: A Global Snapshot
Approximately 40.8 million people were living with HIV worldwide at the end of 2024. About 1.3 million people newly acquired the virus that year, and 630,000 died from HIV-related causes. While those numbers remain significant, they represent substantial declines from the epidemic’s peak. The combination of wider access to testing, earlier treatment, and prevention tools like PrEP continues to reshape the trajectory of the epidemic, though progress is uneven across regions and populations.