HIV is caused by a virus that attacks the immune system, specifically the cells your body relies on to fight off infections. The virus originated in primates in central and west Africa and crossed into humans through contact with infected animal blood, likely during hunting. Today, it spreads from person to person through specific body fluids. There are 1.3 million new infections worldwide each year.
The Virus Behind HIV
HIV stands for human immunodeficiency virus. It belongs to a family of viruses called retroviruses, which work differently from most other infections. Instead of simply hijacking your cells to make copies of itself, HIV actually converts its genetic material into DNA and inserts it permanently into your own cell’s DNA. Once embedded, the virus becomes part of your genetic code in that cell, which is why the body can never fully clear the infection on its own.
The virus particle itself is tiny, roughly 100 nanometers across (about 1,000 times smaller than the width of a human hair). It has an outer fatty membrane studded with protein spikes that act like keys, fitting into specific locks on immune cells. These surface proteins are what allow the virus to latch onto and enter its target: a type of white blood cell called a CD4 cell.
Where HIV Originally Came From
HIV did not appear out of nowhere. It evolved from a closely related virus called SIV (simian immunodeficiency virus) that naturally infects primates in Africa. The two types of HIV in humans came from two different animal sources.
HIV-1, the strain responsible for the vast majority of infections worldwide, jumped to humans from chimpanzees in central Africa. Researchers estimate this crossover happened several hundred years before the virus was identified in the 1980s. HIV-2, a less common and less aggressive strain mostly found in West Africa, came from a smaller monkey called the sooty mangabey. Both crossovers likely happened when hunters were exposed to infected animal blood through cuts or wounds while butchering bushmeat.
HIV-2 is notably different from HIV-1 in two important ways: it spreads less easily between people, and most people infected with it never progress to AIDS. HIV-1, by contrast, will cause AIDS in most untreated individuals over time. When people refer to HIV without specifying a type, they almost always mean HIV-1.
How HIV Gets Into the Body
HIV can only spread through direct contact with certain body fluids from someone who has a detectable level of the virus in their blood. Those fluids are blood, semen (including pre-seminal fluid), rectal fluids, vaginal fluids, and breast milk. Casual contact, saliva, sweat, tears, and sharing food or drinks do not transmit HIV.
For transmission to happen, the virus in these fluids needs a way into the bloodstream. That means contact with a mucous membrane (the moist lining inside the rectum, vagina, mouth, or the opening of the penis), broken skin or open sores, or direct injection through a needle.
The most common routes of transmission are:
- Condomless sex. Receptive anal intercourse carries the highest sexual risk at roughly 1.38% per exposure to an infected partner. Receptive vaginal intercourse is lower, around 0.08% per exposure. Insertive vaginal or anal sex carries lower risk still. Oral sex is possible but extremely rare as a transmission route.
- Sharing needles. Injection drug use with shared equipment carries roughly a 0.63% risk per exposure.
- Blood transfusion. Receiving infected blood is the highest-risk exposure at over 90% per event, though modern screening has made this exceptionally rare in most countries.
- Mother to child. Without treatment, there is roughly a 23% chance of transmission during pregnancy, childbirth, or breastfeeding.
These per-exposure numbers may look small, but they compound with repeated exposures. Factors like higher viral load in the infected person, the presence of other sexually transmitted infections, and open sores or inflammation all increase the odds significantly.
What the Virus Does Once Inside
After entering the body, HIV follows a precise sequence to establish infection. First, the virus’s surface proteins lock onto CD4 receptors on immune cells. The outer shell of the virus then fuses with the cell membrane, releasing its contents inside. Once in, the virus uses a special enzyme to convert its RNA into DNA, which then gets stitched into the cell’s own genetic code. At that point, the infected cell begins producing new copies of the virus, which push out through the cell surface and go on to infect more CD4 cells.
This cycle ramps up quickly. Within two to four weeks of infection, the virus multiplies rapidly and spreads throughout the body. During this acute phase, viral levels in the blood spike to extremely high levels, making the person highly infectious to others. Many people experience flu-like symptoms at this stage: fever, headache, rash, sore throat, and fatigue. These symptoms are easy to dismiss or mistake for another illness, which is one reason HIV often goes undiagnosed early on.
Over months and years, the virus steadily destroys CD4 cells. A healthy person typically has 500 to 1,500 CD4 cells per microliter of blood. As HIV kills these cells faster than the body can replace them, the immune system weakens. Without treatment, most people eventually reach a point where their immune system can no longer defend against infections and cancers it would normally handle easily. That stage is AIDS.
Viral Load and the Risk to Others
Not everyone living with HIV is equally likely to transmit it. The amount of virus circulating in the blood, called viral load, is the single biggest factor determining how infectious someone is. During acute infection, when viral load is at its peak, transmission risk is highest. As the immune system partially controls the virus, the level drops and risk decreases.
Modern antiretroviral treatment can suppress the virus to undetectable levels, defined as fewer than 200 copies per milliliter of blood. At this level, the risk of sexual transmission drops to effectively zero. Three large studies tracking thousands of couples where one partner had HIV found no transmissions when the HIV-positive partner maintained an undetectable viral load. This principle is known as U=U, meaning undetectable equals untransmittable. Research has further shown that even at viral loads up to 1,000 copies per milliliter, the risk of sexual transmission is near zero.
This means that the cause of new HIV infections is not simply the virus existing in someone’s body. It is specifically untreated or inadequately treated HIV, combined with a route of exposure that allows the virus into the bloodstream of another person. Removing either element, through viral suppression, condoms, pre-exposure prophylaxis, or clean needle programs, breaks the chain of transmission.