What reason could there be for some people to doubt that HIV is the causative agent of AIDS?
Koch's postulates have been used to make a connection between particular organisms and specific diseases. However, there are limitations to the use of the postulates. Koch's postulate #3 requires innoculating the host with the potential pathogen. Obviously, this is not possible for diseases only found in humans. This barrier may sometimes be circumvented if there is a good animal model. There is no real good animal model for AIDS. With no good animal model, it is not possible to make the direct connection between organism and disease, and one has to rely on indirect connections.