This chapter provides an overview of the pathogenesis and epidemiology of HIV, highlighting the problematic nature of the virus. The virus poses significant challenges for PLWHA. Understanding how the virus is introduced into the body and the ways that it overcomes the immune functions of the host provides readers with the necessary framework for comprehending the difficulties faced by researchers attempting to create vaccines and efficient antiretroviral medications. The chapter then discusses the various strains of the virus, how they form recombinants and the specific strains found within the provinces in China. The role of STIs and the manner in which they are inextricably linked to, and exacerbate, the ongoing spread of HIV is described. Finally, the chapter provides a rendering of the history of HIV/AIDS globally.