mSTUDY is a 5-year+ research study being conducted at two sites in Los Angeles: The Los Angeles LGBT Center and the UCLA Vine Street Clinic. The purpose of the study is to understand how drug use affects the immune system of HIV-positive and HIV-negative male-identified men who have sex with men (MSM). Given that young, non-white MSM are at greatest risk of HIV infection in the U.S., the study will primarily enroll Latino and African-American/Black men. This project will look at if and how injection drug use affects the likelihood of becoming infected with HIV, and if already HIV-positive, its effects on the progression of the virus. Participants that enroll will have their sexual behavior and social networks assessed through computer questionnaires and will provide various biological samples (blood, urine, saliva, hair/nails, etc.), some of which will be collected to test for sexually transmitted infections (STIs) and substances, and the rest of which will be stored for use in future research. Enrolled participants will come into the clinic for study visits every 6 months for up to 5+ years so we can continue this assessment over time.