The main effect of HIV is the destruction of the immune system. Specifically, the virus targets and kills CD4 T cells, the white blood cells that coordinate your body’s defense against infections. As these cells are steadily depleted over months and years, the immune system weakens to the point where it can no longer protect against infections and diseases that a healthy body would easily fight off. About 40.8 million people worldwide were living with HIV in 2024, and 630,000 died from AIDS-related illnesses that year.
How HIV Destroys Immune Cells
HIV specifically targets CD4 T cells because these cells carry surface receptors the virus can latch onto. The virus binds to a CD4 cell, fuses with its membrane, and slips inside. Once there, it converts its own genetic material into DNA and inserts that DNA directly into the cell’s nucleus, essentially hijacking the cell’s machinery to produce copies of itself. A single infected CD4 cell can produce roughly 10,000 new viral particles during the acute stage of infection.
As new copies of the virus bud off from the host cell, the CD4 cell is destroyed. Those new viral particles then go on to infect more CD4 cells, and the cycle repeats throughout the body. This is what makes HIV so damaging: it doesn’t just attack the immune system, it specifically dismantles the cells responsible for directing the entire immune response.
Why CD4 Loss Is So Devastating
CD4 T cells aren’t ordinary foot soldiers of the immune system. They’re more like commanders. They coordinate both sides of your adaptive immune defense: signaling B cells to produce antibodies and activating killer T cells (CD8 cells) to destroy infected cells. When CD4 counts drop, both of these responses fall apart. Your body loses the ability to recognize and fight off not just HIV itself, but virtually any pathogen it encounters.
The damage extends beyond just losing helper cells. Over time, the killer T cells that remain become exhausted and dysfunctional. The immune system enters a state of chronic activation and inflammation that persists even when the virus is controlled with medication, contributing to problems well beyond what most people associate with HIV.
The Three Stages of HIV Infection
HIV progresses through three distinct stages, each defined by how aggressively the virus is replicating and how much immune damage has accumulated.
Acute Infection
This stage develops within two to four weeks after exposure. The virus multiplies rapidly, and the level of HIV in the blood spikes. Some people experience flu-like symptoms such as fever, headache, and rash. Others notice nothing at all. During this window, the risk of transmitting the virus to someone else is extremely high because of the massive amount of virus circulating in the blood.
Chronic (Latent) Infection
After the initial surge, the virus settles into a quieter phase. It continues replicating at low levels, but many people have no symptoms during this stage. Without treatment, this phase typically lasts around 10 years before the immune system is worn down enough to progress to the final stage, though it can move faster in some people.
AIDS
AIDS is diagnosed when CD4 counts fall below 200 cells per cubic millimeter of blood (a healthy person typically has between 500 and 1,500) or when certain serious infections appear. At this point, the immune system is so compromised that the body becomes vulnerable to opportunistic infections it would normally handle without difficulty. Without treatment, survival at this stage averages about three years.
Opportunistic Infections and AIDS
The infections that define AIDS aren’t exotic diseases. Many are caused by organisms that exist harmlessly in most people’s bodies or environments. They only become life-threatening when the immune system is too weak to keep them in check. The most common categories include bacterial infections like tuberculosis and Salmonella, viral infections like cytomegalovirus, and fungal infections like Pneumocystis pneumonia (PCP) and yeast infections that spread beyond their usual sites.
Certain cancers also become far more likely with severe immune suppression. Kaposi’s sarcoma, several types of lymphoma, and invasive cervical cancer are all classified as AIDS-defining conditions. The CDC’s diagnostic criteria include 26 specific opportunistic infections and cancers that signal the immune system has critically failed.
Effects Beyond the Immune System
Even with effective treatment suppressing the virus, HIV causes chronic low-grade inflammation throughout the body. This persistent inflammation, combined with changes in blood clotting triggered by ongoing immune activation, raises the risk of cardiovascular disease, kidney problems, liver disease, and loss of bone density. These conditions can develop over years and affect people living with HIV at higher rates than the general population.
HIV also affects the brain. The virus crosses into the central nervous system early in infection, hitching a ride inside infected immune cells that pass through the blood-brain barrier. Once inside, it infects the brain’s resident immune cells and triggers inflammation that can damage neurons. This leads to a spectrum of cognitive problems called HIV-associated neurocognitive disorder, or HAND. Symptoms range from subtle difficulties with attention, processing speed, and executive function to, in severe cases, dementia. Even people on effective treatment whose virus is well controlled can show signs of chronic brain inflammation linked to poorer cognitive performance.
How Treatment Changes the Outcome
Antiretroviral therapy works by interrupting the virus’s replication cycle at multiple steps, preventing it from making new copies and infecting fresh CD4 cells. This allows the immune system to recover. Treatment doesn’t cure HIV, because the virus integrates into host cell DNA and can hide in reservoirs, but it can reduce the amount of virus in the blood to undetectable levels.
Life expectancy for people on treatment has improved dramatically. In China’s national treatment cohort, life expectancy at age 20 increased from about 30 additional years in 2013 to nearly 46 additional years by 2023. The gap between people living with HIV on treatment and the general population narrowed from roughly 27 years to about 15 years over that period. Similar trends have been observed globally, though the gap narrows further with early diagnosis and prompt treatment.
When treatment reduces the viral load in blood below 200 copies per milliliter, and a person has been on therapy for at least six months, the virus becomes sexually untransmittable. This principle, known as U=U (undetectable equals untransmittable), has been confirmed in large studies of both same-sex and opposite-sex couples with zero linked transmissions occurring at sustained low viral loads. In the U.S., “undetectable” typically means fewer than 20 copies per milliliter on current tests. During the first six months of treatment, however, the virus can still be present in genital fluids even when blood levels are undetectable, which is why the six-month threshold matters.