HIV (human immunodeficiency virus) is a virus that attacks the immune system, specifically the cells your body relies on to fight off infections. AIDS (acquired immunodeficiency syndrome) is not a separate disease but rather the most advanced stage of an HIV infection, diagnosed when the immune system has been severely damaged. The two terms are related but not interchangeable: a person can live with HIV for decades without ever developing AIDS, especially with modern treatment.
How HIV Affects the Body
HIV targets a specific type of white blood cell called a CD4 cell. These cells act as coordinators of your immune response, signaling other parts of the immune system to spring into action when a threat appears. A healthy person typically has between 500 and 1,600 CD4 cells per cubic millimeter of blood. Over time, untreated HIV destroys enough of these cells that the body can no longer mount an effective defense against infections it would normally handle easily.
The virus works by entering CD4 cells, hijacking their internal machinery to make copies of itself, and then spreading to new cells. Treatment works by interrupting this copying process at various points, which is why people on medication can keep the virus at extremely low levels in their blood.
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, and rash. Not everyone notices these symptoms, and they’re easy to mistake for a regular illness. During this stage the virus is multiplying rapidly and spreading throughout the body, and the level of virus in the blood is extremely high. This makes the acute stage one of the most contagious periods.
Stage 2: Chronic Infection
After the initial burst, HIV settles into a quieter phase sometimes called clinical latency. The virus is still active and multiplying, but at much lower levels. Most people in this stage feel fine and have no noticeable symptoms, which is why HIV can go undiagnosed for years. Without treatment, this stage typically lasts about a decade before the immune system deteriorates enough to progress to AIDS, though it can move faster in some people.
Stage 3: AIDS
A person with HIV receives an AIDS diagnosis when their CD4 count drops below 200 cells per cubic millimeter of blood, or when they develop certain serious infections that a healthy immune system would normally prevent. These are called opportunistic infections and include conditions like a specific type of pneumonia, certain cancers such as Kaposi sarcoma, chronic herpes infections, tuberculosis that has spread beyond the lungs, and severe fungal infections of the throat or lungs. Without treatment, people with AIDS typically survive about three years.
How HIV Spreads
HIV is transmitted through specific body fluids: blood, semen (including pre-seminal fluid), vaginal fluids, rectal fluids, and breast milk. For transmission to occur, these fluids need to enter another person’s body, typically through mucous membranes, broken skin, or direct injection into the bloodstream.
The most common routes are condomless sex, sharing needles or other injection equipment, and transmission from mother to baby during pregnancy, childbirth, or breastfeeding. Blood transfusions and accidental needle sticks in healthcare settings account for a small number of cases. HIV does not spread through saliva, casual contact, or sharing food and drinks under normal circumstances.
One critical point: a person whose treatment has reduced HIV to undetectable levels in their blood cannot transmit the virus sexually. This principle, sometimes summarized as “undetectable equals untransmittable,” has been confirmed in large studies of couples where one partner has HIV and the other does not.
Treatment and What It Means Today
Antiretroviral therapy (ART) is the standard treatment for HIV. These medications block the virus from copying itself at different stages of its replication cycle. The goal is to reduce the amount of virus in the blood to undetectable levels, which for most lab tests means fewer than 20 copies of HIV per milliliter of blood. At this level, the immune system can recover and maintain itself.
The impact of treatment on life expectancy has been dramatic. A large collaborative study published in The Lancet found that a 40-year-old woman who started ART after 2015 could expect roughly 39 more years of life. For men, the estimate was about 37 more years. Those who maintained high CD4 counts (500 or above) had life expectancies only a few years shorter than the general population. This is a far cry from the early decades of the epidemic, when an HIV diagnosis was essentially a death sentence.
Treatment does require consistency. ART is taken daily, and missing doses can allow the virus to rebound and potentially develop resistance to medication. But for people who stay on treatment, HIV becomes a manageable chronic condition rather than a progressive fatal illness.
Prevention Options
Beyond condoms and clean needles, two pharmaceutical prevention strategies now exist. PrEP (pre-exposure prophylaxis) is for people who don’t have HIV but are at risk. Taken as a daily pill or as an injection every two to six months, PrEP reduces the risk of getting HIV from sex by about 99%. For people who inject drugs, it reduces risk by at least 74%. The key is consistency: PrEP becomes much less effective when doses are skipped.
PEP (post-exposure prophylaxis) is an emergency option for someone who may have just been exposed to HIV. It must be started within 72 hours of exposure, and the sooner the better. PEP involves taking medication daily for 28 days.
The Global Picture
As of 2024, approximately 40.8 million people worldwide are living with HIV. About 1.3 million new infections occur each year, and roughly 630,000 people die from AIDS-related causes annually. These numbers represent significant progress compared to the peak of the epidemic, but they also reflect the reality that access to testing and treatment remains uneven across different regions and populations.