I don't believe that a substantial amount if required. You just need the body fluids of an infected individual to come into contact with an open blood vessel in an uninfected person for transmission to occur. This can result whether ten viral bodies are transmitted or if thousands are (comparing the size of viruses, just a drop of a fluid can contain millions). The passage of contaminated body fluids to the bloodstream of an uninfected individual inevitably leads to infection. In the us, those most at risk are non-heterosexual males, iv drug users and their heterosexual partners, and prostitutes. In the us at least, it is not as much of a problem as, for instance, hepatitis. Being in australia, many of the "plagues" the world suffers are either not found or not so common where you are, since it's an island and thereby rather isolated. Based upon my knowledge of the virus and comparing australia with the demography of the us as far as the virus is concerned, I don't think there is much of a problem with it over there. Basically, even the likelihood of you getting the virus through more direct contact is very low (~1-3% for heterosexual intercourse, for example). Basically, no worries concerning your work-related activities thus far.:)
some good, recent news is that scientists are now believing that, once the hiv virus attaches to the cd4 cell (one of the two types of t cells that make up part of your cell-mediated immune reponse) it undergoes a conformational change. If this is the case indeed, a new direction of research will be able to ensue to determine a way to make the virus susceptible to the current vaccines, or to develop a new vaccine to act against the newly-identified structure. 49 volunteers are about to be involved in research in china.