During the spread of human immunodeficiency virus type 1 (HIV-1) in the mucosa, the entire genetic diversity of the viruses is significantly reduced. The vast majority of HIV-1 mucosal infections are established by one or a few viruses and ultimately develop into systemic infections, thus the initial virus is called transmitted/founder virus (T/F virus). The study of T/F virus will benefit understanding its key characteristics resulting in successful viral replication in the new host body, which may provide novel strategies for the development of AIDS vaccines, pre-exposure prophylaxis and other therapeutic interventions. In this review, we summarize the discovery and evolutionary characteristics of T/F virus as well as early immune response after HIV-1 infection, which will establish the basis to explore the features of T/F viruses.