We propose using plasma virome components shared between subjects due to intravenous injection of illicit drugs for early identification of transmission networks in PWID. Transmission studies have shown that HCV precedes HIV infection in PWID, and hence new HCV infections serve as strong predictors of communities at risk for HIV. We now ask if there are virome components that are shared before HCV transmission in PWID. Our hypothesis is that viruses of unknown pathogenicity will accumulate in blood before HIV or HCV infection and that they will be transmitted by injection drug use such that their sequences reveal those epidemiological linkages. By using routinely collected samples, this work could transform the research of drug use networks and change public health surveillance of HIV, HCV, and expose occult dynamics of reinfection and superinfection.