Lecture, three hours; discussion, one hour. Requisites: Life Sciences 7A, 7B, 7C, 23L. Viruses are most common biological entity on planet, and have broad-ranging ecological and evolutionary environmental and medical impacts. This makes understanding roles of viruses central to understanding how ecosystems work, how microorganisms change over time in face of viral infection, how viral pandemics occur and progress, and how many human diseases affect individual health. Students gain comprehensive understanding of how viruses operate in environmental and medical systems, and how they and their hosts evolve over time; and reasoned perspective on future impacts of viruses in changing ecosystems, and on human societies and health. Emphasis on development of mechanistic understanding of viruses. Letter grading.