Microbial coinfection with human immunodeficiency virus-1 (HIV-1), frequently occurring in gastrointestinal, vaginal, and oral mucosal membranes, has been implicated in the disease progression of HIV-1 and acquired immunodeficiency syndrome (AIDS)-related deaths. It has been well established that HIV-1 coinfection with bacteria and other viruses increases the risk of viral transmission, viral replication, and host immune response modulation in HIV-1–infected individuals. In this review, we focus on the interactions between HIV-1 and butyrate-producing anaerobic bacteria under anaerobic conditions, which are primarily found in the gut as well as the vaginal and oral cavity. We have thoroughly examined these interactions, particularly with regard to the upregulation of HIV-1 gene expression and facilitation of viral replication in latently infected cells suspended in butyric acid-containing bacterial culture supernatants. These findings suggest that understanding the molecular mechanisms by which coinfection with HIV-1 and other microbes increase HIV-1 transmission and AIDS progression may provide insights that lead to the development of novel therapeutic strategies and superior prevention methods.