HIV is transmitted when certain body fluids containing infectious virus enter another person's bloodstream or come into contact with susceptible mucous membranes or damaged tissue. The principal fluids involved are blood, semen, vaginal fluids, rectal fluids, and breast milk. The major transmission routes are sexual contact, sharing needles or other injecting equipment, and transmission from a person with HIV to their baby during pregnancy, childbirth, or breastfeeding.
Sexual transmission can occur during anal or vaginal sex, with the probability depending on factors such as viral load, presence of other STIs, mucosal injury, exposure route, and use of prevention measures. HIV is not transmitted through ordinary casual contact such as touching, hugging, sharing household objects, using the same toilet, swimming in the same pool, or sitting near someone. Understanding the specific biological requirements for transmission is essential for distinguishing genuine exposure from situations that pose no realistic risk.
Sexual transmission occurs when HIV-containing genital or rectal fluids come into contact with susceptible mucosal tissue or damaged tissue, or enter the bloodstream. Anal sex carries a higher transmission risk for HIV than vaginal sex because the rectal mucosa is relatively delicate and can develop microscopic lesions that provide access to susceptible cells. Vaginal transmission is also well established and can occur in either direction, although transmission probabilities vary by sexual role and biological circumstances.
Oral sex carries substantially lower HIV transmission risk than anal or vaginal sex, but the precise risk depends on factors such as the presence of blood, oral ulcers, sexually transmitted infections, and ejaculation into the mouth. Effective antiretroviral treatment changes this risk fundamentally: a person with HIV who maintains a confirmed undetectable viral load does not sexually transmit HIV.
Blood exposure is another important route. Sharing needles, syringes, or other equipment used to inject drugs can transmit HIV because contaminated blood can be introduced directly into another person's bloodstream. The same principle explains the historical importance of contaminated blood products before modern screening and why healthcare systems use strict infection-control procedures.
Occupational exposure among healthcare workers can occur through accidental needlestick or sharps injuries, although the absolute risk after a single exposure is low and post-exposure prophylaxis can substantially reduce risk when indicated and started promptly. Tattooing, piercing, or other procedures can present a blood-borne transmission risk if equipment is contaminated and infection-control standards are inadequate. Properly regulated services use sterile or single-use equipment to prevent such transmission.
HIV can be transmitted from a pregnant person to their child during pregnancy, labour, delivery, or breastfeeding. Without preventive treatment, transmission can occur through several mechanisms, including placental exposure, contact with maternal blood and genital secretions during delivery, and virus in breast milk. Antiretroviral therapy during pregnancy and appropriate management during delivery and infant feeding can reduce the risk dramatically. Infants may also receive antiretroviral prophylaxis after birth when clinically indicated. The exact strategy depends on the mother's viral load, treatment history, local guidelines, and feeding circumstances. Prevention of vertical transmission is therefore an important example of how effective antiretroviral medicine can interrupt an otherwise well-established route of HIV transmission.
Viral load is one of the most important determinants of sexual transmission risk. Soon after HIV infection, viral replication can produce a very high viral load, increasing infectiousness. Without treatment, viral load can vary during chronic infection and can rise substantially as immune failure develops. Antiretroviral therapy suppresses viral replication, and sustained viral suppression to an undetectable level prevents sexual transmission. This is the scientific basis of U=U.
Importantly, undetectable refers to the level of virus measured by a validated clinical test and does not mean that HIV has been removed from the body. A person remains infected and normally needs continuing treatment, but when viral suppression is maintained, HIV is not sexually transmitted.
Other sexually transmitted infections can increase HIV acquisition risk because inflammation, ulcers, or disruption of mucosal barriers can make it easier for HIV to enter susceptible cells. Genital ulcers caused by infections such as syphilis or herpes can provide sites through which HIV can cross damaged tissue. Inflammation can also recruit CD4-bearing immune cells to mucosal tissues. Conversely, HIV can affect susceptibility to or the clinical course of other infections because it changes immune function. Prevention of HIV therefore overlaps with broader STI prevention. Condoms, regular testing, prompt treatment of STIs, vaccination against vaccine-preventable infections, and HIV-specific measures such as PrEP and PEP can operate together to reduce overall sexual-health risk.
HIV is not transmitted through saliva, sweat, tears, urine, faeces, or ordinary respiratory droplets under normal circumstances. It is not spread by kissing in ordinary social circumstances, sharing food or drinks, using the same bathroom, touching, hugging, shaking hands, coughing, sneezing, or insect bites. This is because the virus is not efficiently transmitted through these routes and does not reproduce in mosquitoes or other insects.
Deep kissing is not regarded as a meaningful route of HIV transmission, although extremely unusual circumstances involving significant blood exposure in the mouths of both people could theoretically change the exposure conditions. These distinctions matter because fear of casual transmission is medically unfounded and can contribute to stigma against people living with HIV.
Prevention of HIV transmission depends on matching an intervention to the exposure. PrEP protects HIV-negative people when taken according to the prescribed regimen and is highly effective for people with ongoing risk. PEP is used after a potentially significant exposure and should be started as rapidly as possible within the accepted clinical window. Condoms reduce exposure to infectious sexual fluids, while sterile injecting equipment prevents contaminated blood from being transferred.
Testing identifies infection, and immediate treatment reduces viral load and prevents sexual transmission once sustained undetectability is achieved. No single intervention is necessary in every circumstance, and risk assessment should consider the actual exposure, the HIV status and viral suppression of the partner when known, other STIs, and the prevention measures used.
