Inclusion Criteria for All Participants:

• HIV status confirmed by ELISA/Western Blot at screening
• CD4 count greater than 200 cells/mm3 at screening
• Able and willing to communicate in English
• Able and willing to provide adequate information for locator purposes

Inclusion Criteria for Men Practicing Anal-Receptive Sex (Groups 1, 3 and 4):

• Engaged in anal-receptive sex an average of at least once a week in the 2 months prior to study entry
• Agree to refrain from anal intercourse for 24 hours prior to and for one week after sigmoidoscopy

Inclusion Criteria for Men Not Practicing Anal-Receptive Sex (Group 2)

• No history of anal receptive intercourse in the 2 months prior to study entry

Inclusion Criteria for HIV Infected Men (Groups 3 and 4):

• Viral load of either greater than 10,000 copies of RNA/ml plasma or less than 50 copies RNA/ml plasma for at least 2 months prior to study entry
• Have not changed antiretroviral therapy within 6 weeks prior to study entry

Exclusion Criteria:

• For HIV infected patients, 3 or more HSV-2 (herpes) outbreaks in the 12 months prior to screening or 1 or more HSV-2 outbreaks in the 6 months prior to screening
• Active, serious infections (other than HIV) requiring parenteral antibiotic therapy within 15 days prior to screening
• Inflammatory bowel disease (ulcerative colitis or Crohn's disease) or rectal cancer
• Rectal surgery, including fistulectomy
• Diagnosed bleeding disorder, including hemophilia, thrombocytopenia, impaired blood clotting, or current use of anticoagulants that, in the opinion of the investigator, would make participation in the study unsafe or complicate interpretation of study outcome data
• Prosthetic heart valve or diagnosis of valve abnormality
• Hemorrhoid surgery in the 6 months prior to screening
• Bleeding hemorrhoids at screening or in the 6 weeks prior to study entry
• Anal fistulae in the 6 weeks prior to study entry
• Active diarrheal disease (greater than 3 times a day) or bleeding disorder
• Rectal cultures positive for chlamydia or gonorrhea at screening or within 1 month prior to study entry
• Unprotected anal intercourse in the 3 months prior to study entry
• Any other clinical condition or prior therapy that, in the opinion of the investigator, would make the patient unsuitable for the study or unable to comply with study requirements
• Enrolled in any other clinical trial for the duration of their participation in HPTN 056