How are diseases recognized as infectious and how are their causes identified? In this lecture, Dr. Ganem describes how epidemiologists, physicians, and microbiologists work together to identify and study pathogens. He first explains what viruses are and how they reproduce and infect cells. Dr. Ganem then elucidates the increasingly important role of DNA-based techniques in identifying infectious agents by telling how he and other scientists uncovered a virus strongly implicated in causing Kaposi's sarcoma—the leading cancer that affects AIDS patients.