Human immunodeficiency virus and AIDS have received much attention in the last decade. As the major cause of death in young adults, this concern is merited. AIDS has also ushered onto the clinical agenda a number of unusual and interesting pathogens. These have had considerably less attention than the virus itself, but are arguably as important. This book attempts to put into one volume a state of the art review of the basic biology of these opportunistic organisms. When compiling this book, the perceived difficulty was deciding how to restrict the number of chapters, as there is a large number of opportunistic organisms. It soon became clear, however, that the chapters would select themselves, as the work on many of these organisms was still in its infancy. We can be criticized for including Salmonella, which is not thought to be an opportunistic infection, and excluding Crypoccus. Our defence is that the former has an interesting relationship with HIV and the latter is one of those organisms for which the basic biology is still in its early stages. The authors are all active researchers in their respective fields and we are grateful that they managed to review their subjects in the short turn-around time required for a book such as this not to be needlessly outdated on publication. We hope that, if nothing else, this book stimulates interest and more research on these agents: perhaps, in future editions, there will be much more to describe on their molecular biology.