HIV (human immunodeficiency virus) is caused by a virus that attacks the immune system, specifically the white blood cells your body relies on to fight infections. AIDS (acquired immunodeficiency syndrome) is the most advanced stage of HIV infection, occurring when the virus has destroyed enough immune cells that the body can no longer defend itself against diseases it would normally handle easily. About 40.8 million people worldwide were living with HIV in 2024.
How HIV Infects the Body
HIV is a retrovirus, meaning it works backward compared to most viruses. Normally, cells read DNA to make RNA. HIV carries its genetic instructions as RNA, then converts them into DNA once inside a human cell. This reversed process is what makes the virus so persistent.
The virus specifically targets CD4 cells, a type of white blood cell that acts as the coordinator of your immune response. Think of CD4 cells as the generals directing your body’s defense against infections. HIV latches onto the surface of these cells using specific docking points, then slips inside. Once in, the virus converts its RNA into DNA and inserts that DNA directly into the cell’s own genetic code. The infected cell then starts producing copies of the virus instead of doing its job. Those new copies burst out and infect more CD4 cells, repeating the cycle.
This is why HIV is so difficult to eliminate. Because the virus stitches itself into the DNA of immune cells, it essentially hides inside the very system designed to destroy it. Even when treatment suppresses the virus to extremely low levels, that integrated viral DNA remains in some cells indefinitely.
How HIV Spreads Between People
HIV transmits through specific body fluids: blood, semen (including pre-seminal fluid), vaginal fluids, rectal fluids, and breast milk. The virus cannot survive long outside the body and does not spread through casual contact, saliva, sweat, or tears.
The most common route of transmission is unprotected vaginal or anal sex. Not all sexual contact carries equal risk. CDC data puts the per-act risk (without condoms or medication) at roughly 1 in 72 for receptive anal sex, 1 in 909 for insertive anal sex, 1 in 1,250 for receptive vaginal sex, and 1 in 2,500 for insertive vaginal sex. These numbers represent averages. Factors like the presence of other sexually transmitted infections, genital sores, or a higher viral load in the HIV-positive partner can significantly increase the odds.
Other transmission routes include sharing needles or drug injection equipment contaminated with infected blood, and perinatal transmission from mother to child during pregnancy, childbirth, or breastfeeding. In the United States, transmission through blood transfusions or accidental needle sticks in medical settings is extremely rare due to strict screening and safety protocols.
Where the Virus Came From
HIV originated in animals. The virus evolved from SIV (simian immunodeficiency virus), a related virus that infects primates in Central Africa. Scientists believe the jump to humans happened roughly 100 years ago, likely through blood contact when people hunted and handled meat from infected chimpanzees. Once the virus adapted to human cells, it spread through communities long before anyone recognized it as a new disease. HIV wasn’t identified until 1983, decades after it had already been circulating.
The Three Stages of HIV Infection
Without treatment, HIV progresses through three distinct stages over a period of years.
Acute HIV Infection
The first stage develops within two to four weeks after the virus enters the body. During this phase, HIV multiplies rapidly and the viral load in the blood spikes to very high levels. Many people experience flu-like symptoms: fever, sore throat, swollen lymph nodes, rash, muscle aches, and fatigue. These symptoms often get mistaken for the flu or another common illness. This is also the stage when the virus is most easily transmitted to others, because the concentration in bodily fluids is at its peak.
Chronic HIV Infection
After the acute phase, the virus settles into a long, quieter period sometimes called clinical latency. HIV continues replicating, but at much lower levels. Most people feel fine during this stage and may have no symptoms at all, which is why many infections go undiagnosed for years. Without treatment, this stage typically lasts around 10 years before advancing, though it can move faster in some people. Throughout this entire time, the virus is slowly eroding the CD4 cell population.
AIDS
AIDS is the final, most severe stage. It is diagnosed when the CD4 cell count drops to 200 cells per cubic millimeter of blood or below. A healthy immune system typically maintains a CD4 count between 500 and 1,500. At 200 or below, the immune system is severely compromised, leaving the body vulnerable to opportunistic infections and certain cancers that a functioning immune system would normally suppress. These include specific types of pneumonia, tuberculosis, certain fungal infections, and cancers like Kaposi sarcoma. Without treatment, people with AIDS typically survive about three years.
Why HIV Doesn’t Have to Lead to AIDS
The progression from HIV to AIDS is not inevitable. Antiretroviral therapy, taken as a daily medication, can suppress the virus to levels so low that standard lab tests cannot detect it. This is called having an undetectable viral load, defined as fewer than 200 copies of the virus per milliliter of blood. At this level, the immune system can recover and maintain itself, and the person remains healthy indefinitely.
There is also a major public health implication. People living with HIV who maintain an undetectable viral load do not transmit the virus to sexual partners. This principle, known as U=U (undetectable equals untransmittable), has been confirmed through large studies of couples where one partner is HIV-positive and the other is not. Zero transmissions occurred when the HIV-positive partner consistently maintained an undetectable viral load.
Despite effective treatment, HIV remains a global health challenge. In 2024, an estimated 1.3 million people newly acquired the virus worldwide, and around 630,000 people died from AIDS-related illnesses. The gap between those numbers and zero reflects disparities in access to testing, treatment, and prevention tools across different regions and populations.