The risk of becoming infected with HIV depends on the type of exposure:
- The risk for a health care worker after a percutaneous exposure to HIV-infected blood is 0.3 percent; risks may be higher if there is a larger quantity of blood, direct entry of a contaminated needle into an artery or vein, or a deep injury; it may also be higher if the blood is from a terminally ill patient with a very high viral load.
- The risk for a health care worker after mucous membrane exposure to HIV-infected blood is 0.09 percent.
- The risk after sexual assault is unknown but there is a 1/500 chance of a female becoming infected after consensual vaginal intercourse with an HIV-infected male; some males have been found to be "high-risk" HIV transmitters.
- There is a 1/50 chance of becoming infected after anal intercourse.
- The risks are higher if trauma is involved with anal or vaginal intercourse, or if there is exposure to nonintact mucosa.
- Retrospective studies in health care workers have indicated that the risk for HIV infection was reduced by approximately 80 percent by using AZT alone as post exposure prophylaxis.