Bacteria are the microscopic, prokaryotic, single-celled organisms that are few micrometers in length and have different shapes. Bacteria are present in almost all the habitats including the extreme habitats such as radioactive waste, acidic hot springs and the deep biosphere of the Earth's crust. Many species of bacteria are pathogenic and cause infectious diseases such as syphilis, leprosy, cholera, anthrax, bubonic plague, etc. The most common fatal diseases caused by bacteria are respiratory infections like tuberculosis. Despite being pathogenic and causing diseases, there are some useful bacteria as well. For example, only bacteria possess the enzymes and genes required to synthesize vitamin B12 and provide it through the food chain. This book includes some of the vital pieces of work being conducted across the world, on various topics related to role of bacteria in health and disease. It presents researches and studies performed by experts across the globe. The extensive content of this book provides the readers with a thorough understanding of the subject.