Agent-based and phylogenetic analyses reveal how HIV-1 moves between risk groups: Injecting drug users sustain the heterosexual epidemic in Latvia

► We develop an agent-based model to study HIV-1 transmission dynamics. ► We analyze epidemical data of Latvia using the agent-based model and phylogenetics. ► Injecting drug users sustained the heterosexual HIV-1 sub-epidemic in Latvia. ► Heterosexual transmission chain sizes follow a power law distribution. ► Social structure of susceptible population explains observed epidemical dynamics.