This introductory virology course explores the interplay between viruses and their host organisms. The course begins with an overview of how infection is established in a host, then moves to a virologist's view of immune defenses. Next we consider how the replication strategy and the host response determine the outcome of infection, such that some are short and others are of long duration. The mechanisms by which virus infections transform cells in culture are explored, a process that may lead to tumor formation in animals. We then move to a discussion of how viral infections are controlled by vaccines and antiviral drugs. After an introduction to viral evolution, we discuss the principles learned from zoonotic infections, emerging infections, and humankind's experiences with epidemic and pandemic viral infections. The course ends with an exploration of unusual infectious agents such as viroids, satellites, and prions, followed by a discussion of the causative agent of the most serious current worldwide epidemic, HIV-1.