The following "Web lab" simulates the potential spread of HIV through a population of 10 - 30 people. Each participant begins with a virtual sample of body fluid. You will choose the number of HIV+ people within the population. HIV status is assigned randomly by the computer. You will not know who is initially HIV+. A sample of this original fluid, before mixing, is archived by the computer. Each participant will exchange fluid with another participant chosen at random by the computer. The computer will generate three rounds of fluid exchanges and display the results.

Following fluid exchanges, the computer simulates an ELISA (enzyme-linked immunosorbet assay) to determine which fluid samples are now HIV+ and displays the results. Next the detective work begins. Knowing the HIV status of all participants, you should be able to narrow down the list to a few individuals who may have been the original carriers. You can perform an ELISA on fluid archived before the mixes to determine who was initially HIV+.