The US Centers for Disease Control define high risk donors as those that are at higher risk for HIV due to one or more of the following behaviors: men having sex with other men, IV drug use, hemopheliacs who have received human clotting factor concentrates, people who have exchanged sex for drugs or money, people who have had sex with those in the previous categories, people who have been exposed to HIV and jail inmates. You can decide whether or not you will accept a kidney transplant from a person in one of these categories.
The others (older, sicker individuals) are considered Extended Criteria Donors. I chose to *not* accept an ECD kidney but I did take a high risk kidney. The kidney was still screened for HIV with the best tests available but there is a very small risk that I might contract HIV. I did have to get a HIV test at my one year transplant appointment and that came back negative.
Cattlekid, what do you mean by high risk? Was the donor really old or sick or something?