Multinational Observational Cohort of HIV and other Infections (MOCHI). This observational study is to gain information regarding the number of new HIV infections among people who engage in behaviors that make them vulnerable to acquiring HIV across multiple international sites.

Prospective observational cohort study of participants vulnerable to HIV, conducted in two Steps. Step 1 will enroll participants aged 14-55 years who are vulnerable to contracting HIV. They will be evaluated every 12 weeks for HIV. Participants who are diagnosed with HIV will proceed to Step 2, with evaluation of viral load and other HIV-related tests every four weeks for 12 weeks, then 12 weeks later, and lastly 24 weeks later, for a total of 48 weeks.

Step 1 Inclusion Criteria: Female or male aged 14-55 years; Documented negative HIV test according to site-specific HIV testing procedures at entry/screening; Able and willing to provide informed consent; Willing to provide biometric identification; Available for follow-up for the planned study duration; Understands English or the local language as approved by the IRB, including people who are illiterate; Willing to provide contact information for themselves and one personal contact who would know their whereabouts during the study period; Willing to provide information regarding HIV risk behaviors and to undergo testing for HIV and other STIs; Considered to be vulnerable to HIV and other STIs based on any one or more of the following test results and/or self-reported behaviors:

1. Documented history of newly diagnosed syphilis, gonorrhea, chlamydia, Mycoplasma genitalium, herpes simplex virus, or acute hepatitis C virus infection in the 24 weeks prior to screening
2. Self-reported vaginal, oral, or anal intercourse in exchange for money as a regular source of income in the 24 weeks prior to screening
3. Self-reported condomless vaginal or anal intercourse with at least three different partners living with HIV or of unknown status in the 24 weeks prior to screening
4. Self- reported injection drug use (IDU) in the 24 weeks prior to screening
5. Self-reported insertive or receptive anal intercourse with one or more different male or transgender partners in the 24 weeks prior to screening Step 1 Exclusion Criteria: Any significant condition (medical, psychologic/psychiatric or social) that, in the judgment of the study investigator, might interfere with the conduct of the study; Active drug or alcohol use or dependence that, in the judgement of the study investigator, would interfere with adherence to study requirements; Current or past participation in a preventive or therapeutic HIV vaccine study, unless known placebo recipient; Positive β-HCG (human chorionic gonadotropin) pregnancy test (urine or serum) at screening for people of child-bearing potential and people who are amenorrheic for less than 12 consecutive months.

Step 2 Inclusion Criteria: New diagnosis of HIV in the last 90 days during Step 1 as determined by site-specific HIV testing procedures; Able and willing to continue follow-up for the planned study duration Step 2 Exclusion Criteria: Any significant condition (medical, psychologic/psychiatric or social) that, in the judgment of the study investigator, might interfere with the conduct of the study; Active drug or alcohol use or dependence that, in the opinion of the investigator, would interfere with adherence to study requirements

The enrolled study population in step 1 will consist of up to 3000 men and women between 14 and 55 years of age who are vulnerable to HIV and other sexually transmitted infections. The incidence of HIV will be evaluated across all study sites as a primary objective; the incidence of STIs and retention in the cohort will be evaluated as key secondary objectives. During the accrual period, additional volunteers may be screened to replace participants lost to follow-up or withdrawn from the study, but no additional participants will be screened once the accrual period has expired.