My project examines the health effects of feline immunodeficiency virus (FIV) on African lions (Panthera leo) living in Kruger National Park, South Africa. FIV is an immunosuppressive lentivirus related to human and simian immunodeficiency viruses (HIV and SIV, respectively). It has been linked to AIDS in domestic cats, with derangements of immune cells, blood biochemistry, and secondary infections reflective of the related syndrome seen in humans. While FIV is thought to be less severe in wild felid species, its behavior has not been well characterized, and animals living in free-ranging populations are exposed to a diverse assemblage of pathogens that may bear significant costs for host health. My study focuses on immunologic, clinical, and endocrine changes associated with FIV infection in lions. Additionally, it characterizes coinfections with native and non-native parasites and pathogens to examine the importance of FIV for lion health and survival.