Opioid use disorder (OUD) and infectious diseases are intertwined epidemics. In the 1980s, it was realized that human immunodeficiency virus (HIV) could be transmitted via injection drug use. Today, infectious diseases related to opioid use disorder include HIV and hepatitis A, B, and C viruses, as well as bacterial, fungal, and other infections. To reduce the harm of these epidemics, changes must be made to policy and health care practice.