HIV (human immunodeficiency virus) is a virus that attacks the immune system, specifically the white blood cells that protect your body from infections. Without treatment, it gradually destroys these cells until the immune system can no longer fight off diseases. With modern treatment, people living with HIV can expect a normal lifespan and cannot pass the virus to sexual partners.
As of 2024, an estimated 40.8 million people worldwide are living with HIV. About 1.3 million people newly acquired the virus that year, and 630,000 died from HIV-related causes.
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, helping your body recognize and fight off viruses, bacteria, and fungi. HIV latches onto CD4 cells, fuses with them, and hijacks their internal machinery to make copies of itself. A single infected CD4 cell can produce roughly 10,000 new virus particles during the acute stage of infection.
What makes HIV especially difficult is that it inserts its own genetic code directly into the DNA of each CD4 cell it infects. This means the cell essentially becomes a permanent virus factory. As more CD4 cells are destroyed, the immune system weakens, leaving the body increasingly vulnerable to infections it would normally handle without trouble.
HIV vs. AIDS
HIV and AIDS are not the same thing. HIV is the virus. AIDS (acquired immunodeficiency syndrome) is the most advanced stage of HIV infection, diagnosed when the CD4 cell count drops below 200 cells per cubic millimeter of blood. A healthy immune system typically has between 500 and 1,500. At the AIDS stage, the immune system is severely damaged and the body becomes susceptible to serious infections it could previously fight off, known as opportunistic infections.
Not everyone with HIV develops AIDS. With treatment, most people never reach that stage.
The Three Stages of Infection
Acute HIV Infection
This first stage develops within 2 to 4 weeks after infection. The virus multiplies rapidly and spreads throughout the body. Some people experience flu-like symptoms during this period, including fever, headache, and rash, though not everyone does. The level of virus in the blood is extremely high at this point, making transmission risk particularly elevated.
Chronic HIV Infection
Also called clinical latency, this is the long middle stage where the virus continues to reproduce at very low levels. People in this stage often have no symptoms at all and may not know they’re infected. Without treatment, this stage typically lasts around 10 years before progressing to AIDS, though it can be shorter for some people. With treatment, people can remain in this stage for several decades.
AIDS
Without treatment, people diagnosed with AIDS typically survive about 3 years. The immune system is too damaged to protect against infections and certain cancers that a healthy body would control.
How HIV Spreads
HIV is transmitted through specific body fluids: blood, semen, rectal fluids, vaginal fluids, and breast milk. The risk varies dramatically depending on the type of exposure. The CDC estimates risk per 10,000 exposures for different scenarios. Blood transfusion from an infected source carries the highest risk at 9,250 per 10,000. Sharing needles during injection drug use carries a risk of about 63 per 10,000 exposures. Receptive anal intercourse has a risk of about 138 per 10,000, while receptive vaginal intercourse is around 8 per 10,000.
Casual contact does not transmit HIV. Biting, spitting, sharing sex toys, and contact with saliva all carry negligible risk. You cannot get HIV from hugging, shaking hands, sharing food, or using the same toilet.
Testing and Detection
HIV tests vary in how soon they can detect the virus after exposure. The fastest option, a nucleic acid test using a blood draw, can detect HIV as early as 10 to 33 days after exposure. An antigen/antibody lab test from a vein can detect it within 18 to 45 days. A rapid finger-stick version of that same test takes 18 to 90 days. Antibody-only tests have the widest window, taking anywhere from 23 to 90 days.
These time gaps matter. If you test too soon after a potential exposure, the result may come back negative even if you are infected. The period between exposure and reliable detection is called the window period, and it’s the reason repeat testing is sometimes necessary.
Treatment and Life Expectancy
HIV is treated with antiretroviral therapy, commonly called ART. These medications work by blocking the virus at different stages of its life cycle, preventing it from making copies of itself. The goal is to reduce the amount of virus in the blood to undetectable levels.
Starting treatment early makes a significant difference. People who begin ART while their CD4 counts are still relatively high have better immune recovery and longer life expectancy than those who delay. In large clinical trials, people who started treatment immediately had lower rates of both AIDS-related and non-AIDS-related illness compared to those who waited. Some people who delay treatment until their CD4 count drops below 350 never fully recover their immune function, even after a decade on medication.
With consistent treatment, people living with HIV can expect a lifespan similar to people without the virus. Current guidelines recommend starting ART immediately or as soon as possible after diagnosis.
Prevention
One of the most powerful prevention tools is treatment itself. A person living with HIV who maintains an undetectable viral load through treatment has zero risk of transmitting the virus to sexual partners. The CDC has confirmed this principle, often summarized as “Undetectable = Untransmittable” or U=U.
For people who don’t have HIV but are at risk, PrEP (pre-exposure prophylaxis) is a medication taken before potential exposure. It reduces the risk of getting HIV from sex by about 99% when taken as prescribed, and reduces the risk from injection drug use by at least 74%. Condoms also significantly reduce risk during sexual contact.
PEP (post-exposure prophylaxis) is an emergency option taken after a potential exposure. It must be started within 72 hours to be effective and involves a course of medication lasting 28 days.