HIV attacks the immune system’s most important defensive cells, gradually weakening the body’s ability to fight infections and eventually damaging nearly every organ system. The virus specifically targets CD4 cells, a type of white blood cell that coordinates the immune response. A healthy person has between 500 and 1,200 CD4 cells per cubic millimeter of blood. Without treatment, HIV steadily destroys these cells over years, leaving the body vulnerable to infections and cancers it would normally handle with ease.
How HIV Hijacks Immune Cells
HIV cannot reproduce on its own. It needs to get inside a CD4 cell and turn that cell into a virus-making factory. The process starts when the virus locks onto receptors on the CD4 cell’s surface and fuses with its membrane, slipping inside. Once in, the virus converts its own genetic material into DNA and inserts it directly into the cell’s DNA. At that point, the cell’s own machinery starts producing new copies of HIV. Those new virus particles push their way out of the cell, mature into infectious forms, and go looking for more CD4 cells to infect.
This process is ruthlessly efficient. During the earliest stage of infection, a single CD4 cell can produce roughly 10,000 new virus particles. The infected cell is destroyed in the process, and each new particle seeks out another CD4 cell to repeat the cycle. Over time, the body cannot replace CD4 cells as fast as the virus kills them, and the immune system slowly collapses.
The Three Stages of Untreated HIV
Acute Infection
Within two to four weeks of exposure, 50 to 90 percent of newly infected people develop flu-like symptoms: fever, swollen lymph nodes, sore throat, rash, muscle aches, and fatigue. This is the body’s initial immune response to a massive spike in viral activity. Symptoms typically last two to four weeks and then resolve on their own. Many people mistake this for a bad cold or the flu, which is one reason HIV often goes undiagnosed early on. During this window, the virus is replicating explosively and the person is highly infectious.
Clinical Latency
After the acute phase, HIV enters a quieter period sometimes called the chronic or asymptomatic stage. The virus is still active and replicating, just at much lower levels. Without treatment, this stage can last a decade or longer, though it sometimes progresses faster. Many people feel fine during this period and may not know they’re infected. CD4 counts are dropping the entire time, silently eroding the immune system’s reserves.
AIDS
When the CD4 count falls below 200 cells per cubic millimeter, the diagnosis shifts to AIDS. At this point, the immune system is so weakened that the body becomes vulnerable to “opportunistic” infections and cancers that a healthy immune system would easily suppress. These include a specific type of pneumonia caused by a fungus, certain brain infections, chronic intestinal parasites, cancers like Kaposi sarcoma and lymphoma, severe fungal infections in the lungs or esophagus, recurring bacterial pneumonia, and a wasting syndrome that causes dramatic weight loss. Without treatment, people with AIDS typically survive about three years.
Damage Beyond the Immune System
HIV doesn’t just deplete CD4 cells. The chronic inflammation it triggers affects organs throughout the body, even during years when a person feels healthy.
The Gut
One of HIV’s earliest and most damaging strikes happens in the gut. The intestinal lining contains a dense concentration of the exact type of CD4 cells HIV prefers to infect. During the first weeks of infection, the virus rapidly wipes out immune cells in this tissue, particularly the subsets responsible for maintaining the gut’s barrier function and defending against bacteria. Once that barrier is compromised, bacteria and bacterial fragments leak from the intestines into the bloodstream. This “microbial translocation” fuels a state of chronic immune activation that persists for years, even with treatment, and contributes to inflammation-driven damage across the body.
The Heart
People living with HIV face roughly double the risk of cardiovascular disease compared to people without the virus, even after accounting for traditional risk factors like smoking, high cholesterol, and high blood pressure. The chronic inflammation HIV causes appears to accelerate damage to blood vessel walls. Women with HIV are hit particularly hard: they experience a three- to four-fold increase in heart attack risk compared to HIV-negative women, while men with HIV see about a 1.5- to two-fold increase relative to HIV-negative men.
The Brain
HIV can cross into the brain early in the course of infection, and once there, it stays permanently. The virus doesn’t directly infect nerve cells, but it does infect the support cells (called glia) that protect and nourish neurons. This puts brain function at risk over the long term. The result is a spectrum of problems known as HIV-associated neurocognitive disorders, or HAND. Milder forms may involve subtle difficulties with memory, concentration, or processing speed that a person might not immediately notice. More severe forms, which tend to occur in advanced or untreated HIV, can involve significant cognitive decline, behavioral changes, loss of coordination, and difficulty with fine motor tasks like writing or buttoning a shirt.
How Treatment Changes the Picture
Modern antiretroviral therapy works by interrupting the virus’s life cycle at multiple points, blocking it from entering cells, copying its genetic material, integrating into cell DNA, or assembling new virus particles. When taken consistently, these medications can reduce the amount of virus in the blood to undetectable levels. At that point, the immune system can begin to recover, CD4 counts climb back up, and the risk of opportunistic infections drops dramatically.
The impact on transmission is equally striking. A person with an undetectable viral load has zero risk of passing HIV to sexual partners. This finding, confirmed across multiple large studies, is the basis of the public health message “Undetectable equals Untransmittable,” or U=U.
Life expectancy has improved enormously as well, though a gap remains. A study published in The Lancet Public Health tracking people in British Columbia through 2020 found that a 20-year-old male starting treatment could expect to live to about 68, while a 20-year-old female could expect to live to about 61. These numbers represent dramatic gains compared to the pre-treatment era, when an AIDS diagnosis meant survival measured in months. The remaining gap between people with and without HIV is largely driven by the chronic inflammation and organ-level damage that persist even when the virus is well controlled, along with higher rates of smoking, substance use, and barriers to healthcare access that many people with HIV face.
Why Early Diagnosis Matters
Every stage of damage described above, from gut barrier destruction to cardiovascular inflammation to brain involvement, begins earlier than most people realize. HIV enters the brain within weeks. It devastates gut immune tissue during acute infection. Cardiovascular risk accumulates over years of unchecked inflammation. Starting treatment early preserves CD4 cells before they’re destroyed, limits the reservoir of virus that embeds itself in tissues, and reduces the chronic inflammation that drives long-term organ damage. The difference between starting treatment with a CD4 count of 500 versus waiting until it drops to 200 translates into meaningfully better health outcomes across nearly every system in the body.