Transmission

HIV can only be transmitted if sufficient amounts of the virus enter the body:

In people living with HIV who are not receiving ART, the amount of HIV is very high in the blood, in semen, in the liquid film on the mucous membranes of the rectum and the vagina, in breast milk as well as in pus.

Infections then occur mainly during sex without a condom or pre-exposure prophylaxis (PrEP). This is also possible without semen, blood, or other body fluids entering the body, namely through direct intensive contact between mucous membranes (e.g. penis and vagina/uterine cervix or penis and rectum).

HIV can also be transmitted when sharing syringes and needles for drug use. If no protection is provided by HIV therapy, HIV transmission is also possible during pregnancy, birth, or breastfeeding; however, such infections hardly ever occur in Germany.

In medical treatment and nursing care, tattooing and piercing, acupuncture, manicure, and pedicure as well as in first aid, protection is provided by the standard hygiene and protective measures.

HIV is most commonly transmitted during unprotected sex with untreated people with HIV. In this case, the statistical risk of transmission during a single unprotected sexual contact (penetrative/receptive vaginal or anal intercourse) is about one percent. By comparison: Up to 50 percent of unprotected sexual contacts with people infected with gonorrhoea and about 30 percent of unprotected sexual contacts with people infected with syphilis lead to an infection.

The risk increases with the number of such contacts (“cumulative risk”). The risk can also be increased by unfavourable factors, such as when the amount of HIV in the body is very high. Inflammatory sexually transmitted infections increase the risk as well: HIV can leave or enter the body more easily through inflamed mucous membranes.

In people living with HIV who are not receiving ART, additional infected immune cells migrate to the infection sites. They can transmit HIV, thus increasing the amount of HIV in the mucous membranes and the mucus. In HIV-negative people with a sexually transmitted infection, additional immune cells also migrate to the infection sites – they can easily absorb and transmit HIV to other cells.

Anal intercourse without a condom or internal condom, treatment as prevention, or pre-exposure prophylaxis (PrEP) is the sexual practice associated with the highest HIV risk: The rectal mucosa is highly sensitive, which is why minor injuries can easily occur during sex; this risk increases with longer and “rougher” sex. In addition, the intestinal mucosa contains many immune cells that can easily absorb and transmit HIV. In untreated people living with HIV, the amount of HIV in the liquid film on the intestinal mucosa is therefore very high. During receptive anal intercourse, the risk of infection is about two to three times as high as during penetrative anal intercourse.

Vaginal intercourse without a condom or internal condom, treatment as prevention, or pre-exposure prophylaxis (PrEP) is the sexual practice associated with the second highest HIV risk. The risk for the receiving partner is higher than for the penetrating partner: The vaginal mucosa has a larger surface area than the penile mucosa (this area is once again reduced if the penis is circumcised), and semen remains longer in the vagina than vaginal secretion on the penis.

Oral intercourse (sucking or licking the penis, vulva, vagina or anus) only involves an HIV risk for the licking/“blowing” person if large amounts of the virus enter the body through the mouth with semen or blood; even in that case, however, the risk is very low. The oral mucosa is much more stable and resistant than the rectal or vaginal mucosa, fluids containing the virus are diluted by the saliva, and semen does not remain in the mouth as long as in the vagina or rectum. Only a few cases of HIV transmission during oral intercourse have been reported worldwide.

The HIV risk is very high when syringes (especially for drug use) are used by several persons, because the virus directly enters the bloodstream. In some cases, HIV can survive for several days in traces of blood inside the syringe. By contrast, no case of HIV infection through needlestick injury from a discarded needle (e.g. in a playground) has been reported worldwide.

There is also an HIV risk through tattooing and piercing if instruments are used for several persons and are not disinfected.

Blood and blood products (e.g. for surgeries) are very safe in Germany due to various measures (especially testing).

Without protective measures, the risk of transmitting HIV to the baby during pregnancy, birth, and breastfeeding is about 20 percent. Transmission to the baby can be prevented by taking HIV medications and, if necessary, further measures.