The study will last approximately 7 months, with 6 weeks of follow-up. There are four groups in this study. Groups 1 and 2 will enroll HIV uninfected men; Groups 3 and 4 will enroll HIV infected men. Groups 1, 3, and 4 will comprise MSM who engage in anal-receptive sex; Group 2 will comprise men who do not. Patients will provide medical and medication history, undergo a complete physical exam, and receive HIV counseling at screening. An anoscopy (examination of the anus, anal canal, and lower rectum) and blood draw will be conducted at screening, study entry, and Weeks 2 and 4. Rectal secretions will be collected and a sigmoidoscopy (an internal examination of the rectum, distal sigmoid colon, and large bowel using a small camera) will be performed at study entry and Weeks 2 and 4.