Pathogenic bacteria have unique biological properties, which enable them to invade a host and cause sickness. The molecular bases of these biological properties are the determinants of pathogenicity, and the research objectives are to recognize them, identify them chemically and relate their structure to function. Most of our present knowledge comes from studies with cultures in vitro. However, there is a rising interest in bacterial behaviour in the infected host and new methods have been developed for studying it. This book describes those methods and shows how they, and a recent surge in conventional studies, are shedding light on the activities of bacterial pathogens in vivo. It discusses bacterial and host factors that operate in vivo to cause illness, showing how phenomena recognized in vitro relate to behaviour in vivo and, if evidence of relevance is not available now, indicating how it might be obtained.