HIV (human immunodeficiency virus) is a virus that attacks the immune system, specifically destroying the cells your body relies on to fight infections. Left untreated, HIV progressively weakens immune defenses over years and can advance to AIDS, its most severe stage. About 40.8 million people worldwide were living with HIV in 2024, with 1.3 million new infections that year.
With modern treatment, HIV is a manageable chronic condition. People who start medication early and maintain it can expect a life span only a few years shorter than someone without the virus. But understanding what HIV actually does in the body, how it progresses, and how it’s detected and treated makes a real difference in outcomes.
How HIV Attacks the Immune System
HIV targets a specific type of white blood cell called CD4 cells (also known as CD4 T lymphocytes). These cells act as coordinators for your immune response, signaling other immune cells to fight off bacteria, viruses, and fungi. HIV hijacks CD4 cells to make copies of itself, destroying them in the process. During acute infection, a single infected CD4 cell can produce roughly 10,000 new viral particles.
The virus gets into a CD4 cell by first binding to receptors on its surface, then fusing with the cell membrane. Once inside, HIV converts its own genetic material into a form that can be inserted directly into the cell’s DNA. At that point, the cell essentially becomes a factory for producing new copies of the virus. Those new copies push out of the cell, mature into infectious particles, and go on to infect more CD4 cells. This cycle repeats continuously, gradually depleting the immune system’s ability to respond to threats.
The Three Stages of HIV
Stage 1: Acute Infection
Within two to four weeks of infection, many people experience flu-like symptoms: fever, sore throat, swollen glands, rash, muscle aches. This is the body’s initial immune response to the virus. During this stage, the amount of virus in the blood is extremely high, making a person very contagious. Some people mistake these symptoms for a cold or the flu and never get tested, which is one reason HIV spreads undetected.
Stage 2: Chronic (Latent) Infection
After the acute phase, HIV enters a long period where it continues reproducing at lower levels. People in this stage often feel completely fine and may have no symptoms at all, but the virus is still active and still transmittable. Without treatment, this stage can last a decade or longer before the immune system deteriorates enough to progress to AIDS. With treatment, many people stay in this stage indefinitely and never advance further.
Stage 3: AIDS
AIDS is diagnosed when a person’s CD4 cell count drops below 200 cells per milliliter of blood (a healthy count is typically 500 to 1,500) or when they develop specific serious infections. At this stage, the immune system is severely damaged. Without treatment, people with AIDS typically survive about three years. The infections that define AIDS are ones a healthy immune system would normally control easily, which is why they’re called opportunistic infections.
Opportunistic Infections and AIDS Complications
A weakened immune system opens the door to infections and cancers that rarely affect healthy people. Some of the most common include a type of pneumonia caused by a fungus (Pneumocystis pneumonia), a fungal infection of the throat and lungs (candidiasis), tuberculosis, a parasitic brain infection (toxoplasmosis), and a viral eye infection that can cause vision loss (CMV retinitis). Certain cancers are also AIDS-defining conditions, including Kaposi sarcoma (a cancer that causes lesions on the skin and organs), invasive cervical cancer, and several types of lymphoma.
Other complications include chronic severe herpes infections, a brain condition called progressive multifocal leukoencephalopathy, recurring bacterial pneumonia, and HIV-related wasting syndrome, where a person loses significant body weight and muscle mass. The list of recognized AIDS-defining conditions includes more than two dozen illnesses. The good news is that effective treatment prevents most of these from ever developing.
How HIV Is Transmitted
HIV spreads through specific body fluids: blood, semen, pre-seminal fluid, rectal fluids, vaginal fluids, and breast milk. The most common routes of transmission are condomless sex and sharing needles or syringes. HIV does not spread through saliva, sweat, casual contact, sharing food, or mosquito bites.
One of the most important discoveries in HIV prevention is the concept of Undetectable = Untransmittable (U=U). A person living with HIV who takes treatment consistently and maintains an undetectable viral load has zero risk of transmitting the virus to sexual partners. This finding has been confirmed through large studies and is endorsed by the CDC and global health organizations.
Testing and Window Periods
HIV is diagnosed through blood tests, but the type of test determines how soon after exposure it can detect the virus. The fastest option, a nucleic acid test (NAT), can detect HIV 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 done with a finger stick has a window of 18 to 90 days. Standard antibody tests, including most rapid tests and home self-tests, can detect HIV 23 to 90 days after exposure.
If you test too early within these windows, you could get a negative result even if you’re infected. Anyone who thinks they’ve been recently exposed should test at the earliest possible window for their test type and then retest after the full window has passed to confirm.
Treatment and Life Expectancy
HIV is treated with antiretroviral therapy (ART), a combination of medications that block the virus at different points in its life cycle. Some drugs prevent the virus from entering cells. Others block the enzymes HIV uses to convert its genetic material, insert itself into cell DNA, or assemble new viral copies. By attacking the virus from multiple angles simultaneously, ART can reduce the amount of HIV in the blood to undetectable levels.
Treatment is taken daily (or in some cases, as a long-acting injection given every few months) and is lifelong. ART does not cure HIV, but it stops the virus from replicating, allows the immune system to rebuild, and prevents progression to AIDS.
Life expectancy for people on treatment has improved dramatically. A large collaborative study published in The Lancet HIV found that a 40-year-old woman who started ART after 2015 could expect to live an additional 39 years, compared to about 46 years for the general female population. For men, the corresponding figure was 37 additional years. People who maintained high CD4 cell counts (at least 500 cells per microliter) fared even better: women in this group had an estimated 42 additional years and men about 39, bringing life expectancy within just a few years of the general population. Starting treatment early, before the immune system sustains significant damage, is the single biggest factor in achieving these outcomes.
Prevention Options
Beyond condoms and clean needle practices, a medication called PrEP (pre-exposure prophylaxis) is available for people who don’t have HIV but are at higher risk of getting it. PrEP comes as a daily pill or as an injection given every two months, and it significantly reduces the chance of infection. PEP (post-exposure prophylaxis) is an emergency option taken after a potential exposure, though it needs to be started as quickly as possible to be effective.
For people already living with HIV, staying on treatment and maintaining an undetectable viral load is the most effective way to protect partners. Combined with testing, PrEP, and condom use, these tools have made HIV one of the most preventable infectious diseases when the right resources are accessible.