The infectious cycle is what we call the complete course of events in a virus infected cell. In this lecture we discuss the different parts of the infectious cycle and how to study them, including assay of infectious virus particles by plaque assay, multiplicity of infection, and the particle to pfu ratio. Physical methods for detecting viruses include hemagglutination, serological techniques, polymerase chain reaction, and high-throughput sequencing. We show how these methods were used to identify in early 2020 the new pandemic coronavirus SARS-CoV-2. Vincent Racaniello does lecture 2 on the infectious cycle on viruses.