Participants will donate blood or reproductive fluids, or both. From 20 to 150 cc (4 to 30 teaspoonfuls) of blood will be drawn from the arm using a small needle. Participants may be asked to provide blood samples on more than one occasion over the course of the study. No more than 450 cc (less than 1 pint) of blood will be drawn during any 6-week period. Males will be given a private room for semen donation; fluid from females will be collected with a cotton swab after speculum insertion. Participants may also be asked to have a buccal swab. For this procedure, the inside of the cheek is gently scraped with a blunt-ended stick or brush to obtain cells (buccal mucosal cells). The tissues will be used for a variety of studies on the effects of HIV infection on factors that increase or decrease HIV infection.