With bacteria invading our best weapons, where do we begin to search for new weapons? Dr. Finlay showcases three types of bacteria—E. coli, Salmonella, and Listeria—to illustrate how molecular biology is allowing researchers to probe the molecular workings of bacterial infections. He describes how these pathogens use their genetic "toolkits" of virulence factors to cause disease. These factors, as diverse as the hosts they invade, can be transferred between different bacteria by genetic exchange, giving rise to new bacterial pathogens and diseases. Dr. Finlay discusses how an understanding of the role of virulence factors is causing disease can lead to potential new therapies for treatment and prevention.