I mentioned yesterday that one way to help prevent new HIV infections is to treat people who are infected with herpes, another sexually-transmittted virus that infects as much as 20% of the population in the United States. That may seem odd; how does treating one viral infection prevent infection with a second virus?

Studies have shown that, generally, HIV isn’t one of the more highly infectious viruses out there. For example, the hepatitis B and C viruses are transmitted via many of the same routes as HIV (such as sexual activity shared needles), but vary in their infectivity. If, for example, a health care worker gets stuck with a needle from an HIV+ patient, they have a very low probability of becoming infected with the virus; so low that less than 150 occupationally-acquired HIV transmissions have been positively documented in the literature. With hepatitis B virus, however, the risk is orders of magnitude greater: chance of infection following a needlestick is almost 1 in 3. The hepatitis C virus is between them, with about a 1 in 50 chance of becoming infected following such a needlestick.

The reasons for the differences in infectivity vary. Some viruses are present at higher levels in the blood or other body fluids, meaning that more particles are transmitted during each contact. Some have a lower infectious dose, meaning that it takes fewer viral particles to actually start an infection. However, host factors are also involved in transmission of pathogens, and one of these is the presence of other pathogens in the body.

(Continued at AIDS at 25)