AIDS destroys the body’s ability to fight infections. It is the final stage of HIV infection, reached when the virus has killed enough immune cells that the body can no longer defend itself against diseases it would normally handle with ease. At that point, infections that a healthy immune system would suppress become life-threatening.
How HIV Attacks the Immune System
HIV targets a specific type of white blood cell called a CD4 T-cell. These cells act as coordinators of the immune response, signaling other immune cells to attack invaders. A healthy person has roughly 500 to 1,500 CD4 cells per cubic millimeter of blood. HIV hijacks these cells to make copies of itself, killing them in the process.
What makes HIV especially destructive is that the vast majority of CD4 cell death isn’t caused by the virus directly replicating inside them. About 95% of CD4 cells in lymph tissue are in a resting state that doesn’t allow the virus to fully replicate. When HIV tries to infect these resting cells, incomplete fragments of viral DNA accumulate inside them. The cell detects these fragments and essentially self-destructs in a violent, inflammatory form of cell death called pyroptosis. This inflammation then damages surrounding tissue and activates more immune cells, creating a cycle that accelerates the loss of CD4 cells over time.
Only about 5% of CD4 cells are in an active state where HIV can fully replicate. Those cells die quietly after producing new copies of the virus. But the inflammatory chain reaction in the other 95% is what drives the bulk of immune destruction.
What Happens as CD4 Cells Drop
Without treatment, HIV slowly depletes CD4 cells over a period that typically lasts about 10 years, though it can be faster in some people. During most of this time, a person may feel fine and have few or no symptoms. The virus is actively replicating and killing cells, but the body produces new CD4 cells fast enough to partially compensate.
AIDS is formally diagnosed when the CD4 count falls below 200 cells per cubic millimeter, less than a quarter of the low end of normal. At this level, the immune system has critical gaps that leave the body vulnerable to a long list of infections and cancers it would otherwise control. Without treatment, people with AIDS typically survive about three years.
Opportunistic Infections and Cancers
The diseases that define AIDS are called “opportunistic” because they take advantage of the weakened immune system. Many are caused by organisms that live harmlessly in or around most people. With a depleted immune system, these organisms can spread unchecked. The CDC recognizes more than two dozen AIDS-defining conditions, and the most common ones fall into a few categories.
- Lung infections: A fungal pneumonia caused by Pneumocystis jirovecii was historically the most common AIDS-defining illness. Recurrent bacterial pneumonia and tuberculosis are also significant threats.
- Fungal infections: Yeast infections can spread beyond the mouth and throat into the esophagus and lungs. Fungal infections like cryptococcosis can reach the brain and spinal fluid.
- Parasitic infections: Toxoplasmosis, normally kept dormant by the immune system, can reactivate and cause brain abscesses. Chronic intestinal parasites cause prolonged diarrhea lasting more than a month.
- Viral infections: Cytomegalovirus can cause blindness by attacking the retina. Herpes simplex can produce chronic ulcers that persist for weeks.
- Cancers: Kaposi sarcoma, a cancer that causes dark lesions on the skin and internal organs, is closely associated with AIDS. Several types of lymphoma (cancer of the immune system) also occur at much higher rates.
AIDS wasting syndrome is another defining condition: involuntary loss of more than 10% of body weight combined with more than 30 days of diarrhea, weakness, or fever. HIV-related brain disease can also develop, causing confusion, memory problems, and difficulty with coordination.
How Treatment Changes the Outcome
Antiretroviral therapy (ART) works by blocking HIV at multiple points in its life cycle. Some drugs prevent the virus from converting its genetic material into DNA. Others stop that DNA from being inserted into the cell’s chromosomes. A third class prevents new virus particles from maturing into infectious form. Modern treatment combines drugs from different classes, making it extremely difficult for the virus to develop resistance.
When taken daily as prescribed, ART suppresses the virus to undetectable levels in the blood. This allows CD4 cells to recover, often back into the normal range. Even people who have already been diagnosed with AIDS can see their immune function rebuild substantially with consistent treatment. Life expectancy for someone on effective ART now approaches that of the general population.
There is one especially important consequence of successful treatment: a person with an undetectable viral load has zero risk of transmitting HIV to sexual partners. The CDC has confirmed this through large studies tracking thousands of couples. This principle, known as U=U (Undetectable = Untransmittable), has reshaped both prevention strategy and the daily lives of people living with HIV.
Transmission From Mother to Child
Without treatment, the risk of a mother passing HIV to her baby during pregnancy or delivery is 15% to 30%, with an additional 10% to 20% risk from prolonged breastfeeding. With modern antiretroviral treatment that suppresses the mother’s viral load to undetectable levels, transmission rates drop to 1% to 2%. This dramatic reduction is now standard care in most countries with access to these medications.
The Global Picture
As of 2024, approximately 40.8 million people worldwide are living with HIV. Of those, 31.6 million are currently on antiretroviral therapy, representing about 77% of all people living with the virus. The remaining 23% face the greatest risk of progressing to AIDS, largely because of barriers to diagnosis and treatment access in lower-income countries. Where treatment is available and consistent, AIDS-related deaths have dropped sharply, and what was once a terminal diagnosis has become a manageable chronic condition.