Community sample, including HIV infected individuals and their uninfected partners.

Inclusion Criteria for Groups 1, 2, and 3:

• Willing to provide adequate location information
• Planning to remain in the area for the duration of the study

Inclusion Criteria for Group 1:

• HIV-1 antibody positive AND nucleic acid test (NAT) or p24 antigen negative
• Report unprotected receptive or insertive vaginal, oral, or anal intercourse with Group 2 partner on at least 12 occasions (St. Mary's cohort) or 25 occasions (MRC/UVRI Uganda cohort) during the 12 months prior to screening; the first sexual exposure with Group 2 partner must have occurred at least 10 months prior to screening

Inclusion Criteria for Group 2 (St. Mary's cohort):

• HIV-1 infected at screening, as determined by enzyme immunoassay and western blot
• Documentation of HIV-1 infection for at least 12 months prior to screening, as determined by enzyme immunoassay and western blot
• Willing to provide clinical information about HIV-1 RNA and CD4 cell count
• Report unprotected receptive or insertive vaginal, oral, or anal intercourse with Group 1 partner on at least 12 occasions during the 12 months prior to screening; the first sexual exposure with Group 1 partner must have occurred at least 10 months prior to screening

Inclusion Criteria for Group 2 (MRC/UVRI Uganda cohort):

• HIV-1 infected, as determined by enzyme immunoassay and western blot
• Report unprotected receptive or insertive vaginal, oral, or anal intercourse with Group 1 partner on at least 25 occasions during the 12 months prior to screening; the first sexual exposure with Group 1 partner must have occurred at least 10 months prior to screening

• HIV-1 infected for at least 12 months prior to screening, as determined by one of the following criteria:

  1. Report a past medical history indicating Herpes zoster infection within the prior 5 years or report a medical history indicating WHO Clinical Staging Criteria for HIV/AIDS of III or IV within the prior 5 years, OR
  2. CD4 cell count no greater than 450/mm3 for females or no greater than 400/mm3 for males

• Report no prior HIV testing, test HIV-1 seropositive at screening, and meet one of the following criteria:

  1. Report a past medical history indicating Herpes zoster infection within the prior 5 years or report a past medical history indicating WHO Clinical Staging Criteria for HIV/AIDS of III or IV in the prior 5 years, OR
  2. CD4 cell count no greater than 450/mm3 for females or no greater than 400/mm3 for males

Inclusion Criteria for Group 3:

• HIV-1 antibody negative AND nucleic acid test (NAT) or p24 antigen negative
• Test negative for Neisseria gonorrhoeae, syphilis, Chlamydia trachomatis, and trichomoniasis (MRC/UVRI Uganda cohort only)
• Report a monogamous relationship of at least 12 months duration prior to screening

Exclusion Criteria for Groups 1, 2, and 3:

• Injection drug use in the 5 years prior to screening
• Any condition that, in the opinion of the investigator, would be unsafe or interfere with the study
• Any obligations that may require long absences from the area