HIV attacks the immune system’s most critical cells, gradually dismantling the body’s ability to fight infections and disease. The virus specifically targets CD4 T cells, a type of white blood cell that coordinates immune responses. 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, leaving the body vulnerable to infections it would normally handle with ease.
How HIV Hijacks Immune Cells
HIV doesn’t just kill CD4 cells. It turns them into virus factories. The process starts when the virus latches onto receptors on a CD4 cell’s surface, then fuses with the cell membrane to get inside. Once in, HIV converts its own genetic material (RNA) into DNA using a special enzyme, then inserts that DNA directly into the cell’s own genome. At that point, the cell’s normal machinery starts producing copies of the virus instead of doing its job. Those new copies burst out and go on to infect more CD4 cells, repeating the cycle.
This is what makes HIV so difficult to eliminate. Because the viral DNA becomes part of the host cell’s DNA, the infection is permanent. Even when treatment suppresses the virus to undetectable levels, those integrated copies remain hidden in long-lived cells, ready to reactivate if treatment stops.
The Three Stages of Infection
Acute Infection
Within two to four weeks of exposure, HIV multiplies rapidly and spreads throughout the body. During this burst, the virus destroys large numbers of CD4 cells and the level of virus in the blood spikes. Some people develop flu-like symptoms: fever, headache, rash. Many don’t notice anything unusual. This stage is also when a person is most contagious, because of the extremely high amount of virus circulating in the blood.
Chronic Infection
After the initial burst, the immune system partially controls the virus, and HIV settles into a quieter phase. The virus continues replicating at low levels, often producing no noticeable symptoms for years. Without treatment, this stage typically lasts about a decade before progressing, though it can move faster in some people. With antiretroviral therapy, people can remain in this stage for several decades.
AIDS
AIDS is diagnosed when CD4 counts drop below 200 cells per cubic millimeter, or when certain serious infections develop. At this point, the immune system is severely damaged. Without treatment, survival averages about three years.
Damage Beyond CD4 Cells
HIV doesn’t limit its destruction to circulating immune cells. The virus inflicts serious damage on the lymph nodes, the organs where immune responses are organized. In chronic infection, lymph nodes become enlarged and chronically inflamed. This inflammation traps large numbers of immune cells in the tissue, exposing them to intense local concentrations of virus and inflammatory signals. Many of these cells die as bystanders, killed not by direct infection but by the hostile environment. Over time, this disrupts the architecture of the lymph nodes themselves, impairing the body’s ability to mount coordinated immune responses of any kind.
The result is broad immune dysfunction. Both the cellular side of immunity (T cells hunting infected cells) and the antibody side (B cells producing targeted proteins) become dramatically impaired. B cells go into overdrive, producing antibodies indiscriminately rather than in targeted fashion, a sign of immune chaos rather than effective defense.
Gut Damage Starts Early
The gastrointestinal tract contains a huge proportion of the body’s immune cells, and HIV hits it hard. During acute infection, the virus massively depletes CD4 cells in the gut lining. This destruction damages the intestinal barrier, increasing intestinal permeability up to fivefold compared to healthy individuals. The gut lining develops structural changes: the finger-like projections that absorb nutrients shrink and flatten, while the tissue remodels in ways that further compromise its function.
A leaky gut allows bacterial products to cross into the bloodstream, fueling chronic inflammation throughout the body. This gut damage begins in the earliest days of infection and persists even after treatment brings the virus under control, contributing to long-term health problems that extend well beyond the immune system.
Effects on the Heart and Blood Vessels
People living with HIV face 1.5 to 2 times the risk of cardiovascular disease compared to HIV-negative individuals. This elevated risk persists even with effective antiviral treatment. The primary driver is chronic inflammation. Even when the virus is fully suppressed, low-grade immune activation continues, accelerating the buildup of plaque in arteries. The inflammatory signals that HIV triggers throughout the body damage blood vessel walls over time, making heart attacks and strokes more likely at younger ages than would otherwise be expected.
Effects on the Brain
HIV enters the brain by riding inside infected immune cells, particularly monocytes and CD4 T cells, that cross the blood-brain barrier. Once inside, the virus and the inflammation it generates can cause a range of neurological problems grouped under the term HIV-associated neurocognitive disorders. Symptoms include slowed thinking and movement, difficulty with focus and memory, depression, irritability, and reduced motivation. These cognitive effects can occur even when the virus is well controlled with treatment, though they tend to be milder in that context.
Skin Conditions at Different Stages
Skin problems are among the most visible effects of HIV, and they tend to worsen as CD4 counts decline. At relatively higher CD4 levels, people may develop more severe versions of common conditions like psoriasis or seborrheic dermatitis. As counts drop below 250, an intensely itchy rash called eosinophilic folliculitis can appear on the face, chest, and upper back.
Below 200, the risk of widespread molluscum contagiosum increases, with lesions that can become large and disfiguring, particularly on the face. Below 100, the picture becomes more serious: chronic, deep herpes ulcers that don’t heal, vascular skin lesions from bacterial infections, and Kaposi’s sarcoma, a cancer that produces distinctive brown, red, or purple patches and nodules on the skin. These conditions are strong signals of advanced immune suppression.
Opportunistic Infections by CD4 Count
As CD4 counts fall, specific infections emerge at predictable thresholds. This progression is one of the clearest illustrations of what HIV does to the body’s defenses:
- Below 200: Pneumocystis pneumonia (PCP), a fungal lung infection that was one of the first recognized signs of the AIDS epidemic, becomes a significant risk.
- Below 150: Certain fungal infections like histoplasmosis become dangerous, particularly in regions where these fungi are common.
- Below 100: Toxoplasmosis, a parasitic brain infection, threatens anyone who carries the dormant parasite (which many people do without knowing).
- Below 50: Mycobacterium avium complex (MAC), a bacterial infection that can spread through multiple organs, becomes a risk in people not on effective treatment.
Each of these infections is caused by organisms that a healthy immune system keeps in check effortlessly. Their appearance marks specific milestones of immune collapse.
Life Expectancy With Treatment
Modern antiretroviral therapy has transformed HIV from a death sentence into a manageable chronic condition. Treatment works by suppressing viral replication to the point where the virus becomes undetectable in blood tests, generally below 20 copies per milliliter. At that level, the virus cannot be transmitted sexually, a principle known as U=U (Undetectable equals Untransmittable).
A large collaborative study published in The Lancet HIV, covering more than 200,000 people across Europe and North America, found that a 40-year-old woman who started treatment after 2015 could expect to live an additional 39 years, reaching age 79. For men, the estimate was 37 additional years, reaching age 77. These numbers approach general population life expectancy and represent a dramatic improvement over earlier eras of treatment. Starting treatment with a higher CD4 count makes a substantial difference: people who began with very low counts (under 50) had roughly 15 fewer years of expected life remaining compared to those who started earlier.
The critical factor is early diagnosis and prompt treatment. When someone begins therapy before significant immune damage has occurred, the body can rebuild much of its CD4 cell population and maintain effective immune function for decades. The longer HIV goes untreated, the more irreversible damage accumulates in the gut, lymph nodes, blood vessels, and brain.