Building on the biology and epidemiology of HIV infection presented in earlier chapters, Chapter 7 discusses two important topics: the biological bases of HIV transmission and modes of HIV transmission.
The first section on the bases of HIV infection emphasizes that there is still no cure for AIDS, so the prevention of HIV infection is critical for containing the epidemic in developed and developing countries. These efforts require that the factors influencing HIV transmission be understood. The evidence for assigning risks to different activities comes from two primary sources: theoretical biological considerations and epidemiological studies. Several specific examples of how theoretical biological considerations and epidemiological studies together can be used to make predictions of risk for HIV transmission are discussed.
Following this introductory section, the sources of infectious HIV in an infected individual are discussed. Semen, blood, and vaginal secretions are identified as the body fluids containing the highest levels of HIV, whereas breast milk has lower but significant levels of infectious virus. In contrast, saliva, urine, tears, sweat, and feces have low or no detectable levels HIV. It is pointed out that infected cells such as macrophages are also a source of infectious virus so that contact with body fluids or tissues containing HIV-infected cells may lead to transmission of HIV.
A discussion of the stability of HIV is then presented and the reader is reminded that outside the body, HIV is very fragile and rapidly becomes inactivated when exposed to the drying effects of air and light or is brought into contact with soap and water. It is pointed out that the sources of HIV in body fluids and the fragility of the virus explain why HIV is not transmitted by casual contact. The fact that the cellular targets for HIV infection include macrophages and helper T cells and that macrophages are found in the mucosal linings of the throat, anus, and genital tract leads to a discussion of the limitations of theoretical considerations for predicting the risk of HIV transmission. Based on the fact that the lining of the oral cavity does contain macrophages, it might have been thought that unprotected oral sex would have a high potential for HIV transmission, when in reality the epidemiological data suggests the risk is much lower than unprotected anal or vaginal sex.
The last section of this chapter discusses modes of HIV transmission in detail. The meaning of casual contact is spelled out with many examples of everyday non-sexual contact and it is clearly stated that there is no measurable risk of transmission. Perinatal transmission from mother to unborn child is discussed and it is noted that without antiviral treatment there is an approximately 30% chance of the gestating child being infected. The risk of HIV infection through a blood transfusion is discussed in some detail and it is pointed out that since screening of blood products has been introduced the risk of receiving HIV-infected blood has dropped to 2.25 in 100,000. Transmission of HIV through the use of contaminated needles by injection drug users is briefly mentioned and the issue of accidental needle sticks by health workers is discussed. It is pointed out that the rate of transmission of HIV by needle sticks is quite low (0.3%) when compared to transmission of hepatitis B (50%).
Finally, the risk of HIV transmission during intimate sexual contact with an HIV-infected individual is considered. It is recognized that there is a spectrum of risk associated with sexual practices ranging from the most risky- unprotected receptive anal sex, to the least risky- abstinence. This potentially controversial issue is dealt with in a non-judgmental manner, recognizing that risks cannot be completely quantified for less risky practices such as oral sex. The section ends with a discussion of the effectiveness of condoms for preventing the transmission of HIV during sexual intercourse and provides instruction on how to use condoms correctly. The reader is reminded that it is the individual’s personal responsibility to make decisions on his or her own sexual practices and to minimize the risk of HIV infection.
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