Virulence: The ability of an agent of infection to produce disease. The virulence of a microorganism is a measure of the severity of the disease it causes. SLIDESHOW Heart Disease: Causes of a Heart Attack See Slideshow From Healthy Resources 10 Rheumatoid Arthritis SymptomsWhat Is Exocrine Pancreatic Insufficiency? Featured Centers Good and Bad Foods for PsoriasisVideo: Getting Personal on Life With MS Health Solutions From Our Sponsors Shot-Free MS Treatment Your Child and COVID-19