How does a tiny virus wreak havoc on the human body to create modern-day scourges such as AIDS? Noted virologist and AIDS expert Dr. Donald Ganem offers a comprehensive primer on viruses, explaining how they reproduce and infect cells, and why it often takes a team of scientists and physicians to identify pathogens that cause disease. Ganem also describes the important role of DNA-based techniques in identifying infectious agents. From the Series:HHMI’s Understanding Biomedical Research Series