he transmission of an infectious agent from an animal to a human being initiates a series of events that constitute the pathogenesis of the infection. Pathogenesis is the entry, primary replication, spread to target organs, and establishment of infection in the target organs. The process by which a pathogen replicates itself in the human host depends on cell-specific and organ-specific receptors, cell and tissue injury, and host immunity and other defense factors. The final outcome is either termination of infection, persistence and latency of infection, transmission to another host, or some combination of these. This series of events is not specific for zoonotic infections, except possibly that zoonotic agents are rarely sexually transmitted. The zoonoses, however, illustrate some of the more interesting and complex patterns that have evolved in nature.