Infectious diseases are the diseases caused by infectious organisms like bacteria, viruses, fungi, worms, or helminths. In cases where the immune system of the host is fully active, the host does not develop any disease. However, if the host's immune system is weak or if the infectious agent is able to overwhelm the host's immune system, the host develops infectious disease. Infectious diseases are a cause of concern because they are a major cause of mortality around the world, and pose challenges related to health, society, and economy. In case of infectious diseases, various evolutionary aspects, such as, pathogens evolving mechanisms to evade host defense, developing drug resistance, pathogen's adaptation to host environment, competition between the pathogen and the host microbiota, evolving virulence, and transmission into the new hosts, need to be considered. Researchers could develop effective prevention and control strategies if they are equipped with a better understanding of the key evolutionary features of infectious diseases, such as pathogenicity, infectiousness, and transmissibility. All such evolutionary features are usually revealed through genome structures and organization, population dynamics, and host/vector physiological systems. This book presents researches and studies performed by experts across the globe. Those in search of information to further their knowledge will be greatly assisted by this book.