HIV is a virus that infects and gradually destroys the immune cells your body relies on to fight infections. It specifically targets a type of white blood cell called a CD4 T cell, hijacking its internal machinery to produce new copies of itself. Over years, this process depletes CD4 cells to the point where the immune system can no longer defend against infections it would normally handle easily.
Understanding how HIV operates, step by step, also explains why modern treatment works so well and why the virus is so difficult to eliminate completely.
How HIV Enters a Cell
HIV can’t reproduce on its own. It needs to get inside a living human cell, and it’s extremely specific about which cells it targets. The virus carries proteins on its outer surface that lock onto a receptor called CD4, found mainly on a subset of immune cells called helper T cells. But attaching to CD4 alone isn’t enough. The virus also needs to grab a second receptor, either CCR5 or CXCR4, on the same cell surface. Different strains of HIV prefer one co-receptor over the other, which partly determines which cells get infected and how the disease progresses.
Once HIV has latched onto both receptors, its outer membrane fuses directly with the cell membrane, and the viral core slips inside. This fusion step is one of the key targets for certain HIV medications that work by physically blocking the virus from docking.
Turning Viral RNA Into DNA
HIV stores its genetic instructions as RNA, but human cells read DNA. So the virus carries its own enzyme, called reverse transcriptase, that converts its RNA into DNA. This process begins within one to two hours of the virus entering the cell.
Reverse transcriptase is fast but sloppy. It makes frequent errors when copying the viral genetic code, which means every new generation of HIV is slightly different from the last. This built-in mutation rate is one reason the virus can develop resistance to drugs and why a single medication is never used alone.
Once the viral RNA has been converted into a DNA copy, another viral enzyme called integrase splices that DNA directly into the cell’s own chromosomes. At this point, the viral instructions are permanently woven into the cell’s genome. Every time that cell divides, it copies the viral DNA right along with its own. The cell has, in effect, become a permanent carrier.
Making New Virus Particles
With its genetic code embedded in the host cell’s DNA, HIV can now use the cell’s own protein-building machinery to produce viral components. The cell reads the viral instructions and churns out long chains of viral proteins, essentially building parts on an assembly line it didn’t choose to run.
These protein chains and copies of viral RNA migrate to the cell’s outer membrane, where they assemble into new virus particles. The immature virus pushes outward through the membrane in a process called budding, wrapping itself in a piece of the cell’s own outer layer as it exits. This stolen membrane becomes the virus’s envelope.
At this stage, the new particle isn’t yet infectious. A viral enzyme called protease must cut the long protein chains into their final, functional pieces. This step, called maturation, transforms the internal structure of the virus into its characteristic cone-shaped core. Only after maturation is the virus capable of infecting a new cell. The rates at which protease cuts different protein sites vary by up to 400-fold, creating a precise sequence of structural changes that ultimately produces an infectious particle. Protease inhibitors, one of the major classes of HIV medication, work by blocking this final step, leaving the virus structurally incomplete and unable to infect anything.
How HIV Destroys the Immune System
The damage HIV causes goes far beyond the cells it directly infects. Only about 5% of CD4 T cells in a given tissue are in the activated state that allows productive infection. When the virus successfully infects one of these active cells, it hijacks the cell to produce new viruses, and the cell dies quietly through a process called apoptosis.
But the remaining 95% of CD4 T cells are resting cells that don’t support full viral replication. When HIV enters these cells, infection stalls partway through, leaving incomplete fragments of viral DNA floating in the cell’s interior. The cell detects these foreign DNA fragments and triggers a dramatically different response: pyroptosis, a highly inflammatory form of cell death. The dying cell essentially sounds an alarm, releasing signals that attract more immune cells to the area, which in turn become new targets for the virus. This creates a destructive cycle where the immune system’s own response to the virus accelerates its spread.
HIV also kills bystander cells that were never directly infected. Infected cells release viral proteins that can trigger death in neighboring CD4 cells. Over time, this combination of direct killing, inflammatory death, and bystander destruction steadily erodes the immune system’s capacity.
Stages of HIV Infection
HIV infection typically unfolds in three stages. During acute infection, the first few weeks after exposure, the virus replicates explosively. Viral levels in the blood spike, making this the period of highest transmission risk. Many people experience flu-like symptoms during this phase, though some notice nothing at all.
The second stage, chronic infection, can last a decade or longer without treatment. The virus continues multiplying, but at much lower levels. A person may feel fine during this period, but CD4 cells are gradually declining. With effective treatment, most people remain in this stage indefinitely and never progress further.
Without treatment, CD4 counts eventually drop below 200 cells per cubic millimeter of blood (a healthy count is roughly 500 to 1,500). At that point, or when certain serious infections appear, the diagnosis changes to AIDS. At this stage, the immune system is too weakened to fight off infections that a healthy body would suppress easily.
Why Treatment Works but Can’t Cure
Modern HIV treatment uses combinations of drugs that block the virus at different stages of its life cycle. Some prevent the virus from converting its RNA to DNA. Others block the integrase enzyme so viral DNA can’t splice into human chromosomes. Protease inhibitors stop new virus particles from maturing into their infectious form. By hitting multiple steps at once, these drug combinations reduce the amount of virus in the blood to undetectable levels.
A person with an undetectable viral load cannot sexually transmit HIV. This principle, known as Undetectable equals Untransmittable (U=U), is backed by large clinical studies and endorsed by major public health agencies worldwide.
But treatment cannot eliminate the virus entirely, and this is because of a biological feature called the latent reservoir. When HIV integrates its DNA into a resting CD4 cell, that cell may sit dormant for years, neither producing new virus nor displaying any signs of infection on its surface. The immune system doesn’t recognize these cells as threats, and because the virus isn’t actively replicating inside them, medications have no effect. These silent carriers persist throughout the body, including in the central nervous system, lymph nodes, and genital tract. If a person stops taking their medication, these dormant cells can reactivate and begin producing virus again, causing viral levels to rebound.
How HIV Spreads Between People
HIV transmission begins at mucosal surfaces, the thin tissue lining the genital tract, rectum, and mouth. When the virus reaches these surfaces, it can cross the tissue barrier through several routes, though none are particularly efficient. Studies using cultured genital tissue found that less than 0.02% of an initial amount of virus successfully passed through the epithelial layer directly. The low per-exposure transmission rate of HIV reflects this biological bottleneck.
What makes transmission possible are the immune cells stationed just beneath the mucosal surface. Dendritic cells, including a type called Langerhans cells, patrol this area and can capture the virus. In human skin tissue studies, Langerhans cells accounted for more than 95% of HIV spread from the initial site of exposure. These cells carry the virus to nearby lymph nodes, where they pass it to the large populations of CD4 T cells concentrated there. Once the virus reaches the lymph nodes, robust replication begins and infection is established.
How Testing Detects the Virus
Different HIV tests look for different biological signals, and the type of test determines how soon after exposure it can detect infection. The earliest marker is a viral protein called p24, which appears in the blood before the immune system has produced any antibodies. Lab-based tests that look for both p24 antigen and antibodies using blood drawn from a vein can detect HIV as early as 18 to 45 days after exposure. The same type of test done with a finger stick has a window of 18 to 90 days.
Nucleic acid tests, which look for the virus’s genetic material directly, can detect infection as early as 10 to 33 days after exposure, making them the fastest option. Antibody-only tests, which include most rapid tests and home self-tests, require 23 to 90 days to produce a reliable result. Testing before these windows have passed can produce a false negative even if the virus is present.