HIV attacks the immune system by infecting and destroying the very cells your body relies on to fight off infections. Specifically, it targets CD4 T cells, a type of white blood cell that coordinates your immune response. A healthy person has between 500 and 1,500 CD4 cells per cubic millimeter of blood. Left untreated, HIV steadily destroys these cells over years, eventually leaving the body unable to defend itself against infections and diseases it would normally handle with ease.
How HIV Hijacks Your Immune Cells
HIV doesn’t just kill CD4 cells. It turns them into factories for making more virus. The process starts when the virus latches onto receptors on the surface of a CD4 cell and fuses with its outer membrane to get inside. Once in, HIV converts its own genetic material into a form that can be spliced directly into the cell’s DNA. At that point, the cell’s own machinery starts producing new copies of the virus.
A single infected CD4 cell can produce roughly 10,000 new viral particles. Those new copies push their way out of the cell, destroying it in the process, then spread to infect more CD4 cells. This cycle repeats continuously. Without treatment, the virus slowly wins a war of attrition against the immune system, killing CD4 cells faster than the body can replace them.
The Three Stages of Infection
Acute Infection
The first stage typically develops within 2 to 4 weeks after exposure. During this phase, the virus is multiplying rapidly and spreading throughout the body. Many people experience flu-like symptoms: fever, sore throat, swollen lymph nodes, rash, muscle aches, and fatigue. These symptoms can be easy to dismiss as a regular illness, which is one reason HIV often goes undiagnosed early on. The viral load in the blood is extremely high during this window, making the infection highly transmissible.
Chronic (Latent) Infection
After the initial spike, the virus settles into a quieter phase. It continues replicating, but at much lower levels. Most people feel fine during this stage and may have no symptoms at all, which is why it’s sometimes called clinical latency. Without treatment, this phase typically lasts about 10 years, though it can be shorter for some people. Throughout this entire time, the virus is still active, gradually chipping away at the CD4 cell population.
AIDS
AIDS is diagnosed when CD4 counts drop below 200 cells per cubic millimeter or when certain serious infections appear. At this point the immune system is severely damaged. Without treatment, people with AIDS typically survive about 3 years. The cause of death is rarely HIV itself. Instead, it’s the infections and cancers that take hold once the immune system can no longer mount a defense.
What Happens When the Immune System Fails
As CD4 counts fall, your body becomes vulnerable to opportunistic infections, diseases caused by bacteria, viruses, fungi, and parasites that a healthy immune system would normally keep in check. Some of these can begin appearing when CD4 counts drop below 500, though the greatest risk comes below 200.
The most common opportunistic infections include:
- Pneumocystis pneumonia (PCP): a serious lung infection caused by a fungus that rarely affects people with healthy immune systems
- Tuberculosis: a bacterial infection that primarily affects the lungs but can spread to the kidneys, brain, skin, and lymph nodes
- Candidiasis (thrush): a fungal infection that can develop in the mouth, throat, lungs, or esophagus
- Toxoplasmosis: a parasitic infection that can cause brain inflammation
- Herpes simplex infections: which can become severe and affect the eyes, brain, throat, and genitals
- Salmonella infection: a bacterial illness that targets the intestines and can become chronic
Effects on the Brain and Nervous System
HIV can enter the brain early in the course of infection, and once there, it stays for life. The virus doesn’t directly infect nerve cells, but it does infect supporting cells called glia that protect and nourish neurons. This sets off chronic inflammation in the brain that, over time, can cause real damage.
HIV-related brain inflammation can lead to confusion, forgetfulness, difficulty concentrating, behavior changes, headaches, anxiety, and depression. Some people develop coordination problems or difficulty walking. Imaging studies show that HIV infection can actually cause parts of the brain involved in learning and information processing to shrink. Even in people on effective treatment, low levels of viral material integrated into brain cells can continue driving this inflammation, contributing to a long-term neurodegenerative process.
Chronic Inflammation and Heart Disease
One of HIV’s less obvious effects is the persistent, low-grade inflammation it triggers throughout the body. Even when treatment suppresses the virus to undetectable levels, people with HIV still have higher markers of systemic inflammation compared to people without the virus. This ongoing inflammatory state takes a toll on blood vessels and the heart.
People living with HIV face an elevated risk of cardiovascular disease. This comes from a combination of factors: the chronic inflammation itself, a higher prevalence of traditional risk factors like high cholesterol and high blood pressure in this population, and the metabolic side effects of some HIV medications. Standard cardiovascular risk calculators designed for the general population don’t account for these HIV-specific factors, which means the actual risk can be higher than predicted.
How Treatment Changes the Picture
Modern antiretroviral therapy works by interrupting the virus’s life cycle at different stages, blocking it from entering cells, converting its genetic material, integrating into cell DNA, or assembling new copies. When taken consistently, these medications reduce the amount of virus in the blood to undetectable levels and allow CD4 counts to recover.
The difference treatment makes is dramatic. A large study of more than 206,000 people found that those who started treatment with high CD4 counts had a life expectancy only a few years shorter than people without HIV. This represents an enormous shift from the early years of the epidemic, when an HIV diagnosis was effectively a death sentence. Treatment also reduces, though doesn’t eliminate, the chronic inflammation that drives complications like heart disease and brain changes.
The key variable is timing. People who begin treatment early, before their CD4 counts have dropped significantly, get the best outcomes. Those who start treatment later, after substantial immune damage has already occurred, face a harder road to recovery. The immune system can rebuild itself to a remarkable degree, but the longer the virus runs unchecked, the more lasting damage it does to the body’s ability to fight infection, protect the brain, and maintain healthy blood vessels.