If you ingest a drop of blood from a woman who is menstruating, will the enzymes in your saliva render HIV harmless?

Answer

Viruses are present in the blood of an HIV-infected person, certainly without treatment. An albeit smaller amount of virus can also be found in cervical/vaginal fluid, which will increase when menstrual blood is added.

The saliva contains inhibitory factors that can reduce the risk of infection when virus enters the mouth (e.g. via contaminated bodily fluids). Especially in the absence of (bleeding) injuries in the mouth, the risk of transmission of infection is estimated to be extremely low. Large amounts of virus (e.g. when large amounts of contaminated blood are ingested) continue to pose a risk.

The chance that someone is infected with HIV depends on exposure, not only sexually but also through unsafe use of (drug) needles, transfusions in the distant past or in unsafe countries, possibly also through mother-child transmission.

Safe sex involves the use of a condom, which not only protects against transmission of HIV, but also against many other infections, eg some hepatitis viruses and also cervical cancer-causing viruses. Its use is always appropriate. An exception can be made for people who have each other as their only partner and who have never had another partner in life with unprotected sex or who have had another exposure to infection. When in doubt (eg with a view to fathering a child), you can have yourself tested.