This book provides knowledge on the diversity of disease-causing microbes including bacteria, viruses, fungi and protozoans—the major diseases they cause and the environmental factors that influence the occurrence, transmission and spread of microbes along with the epidemiological factors determining the occurrence of disease. The book mainly covers a broad spectrum of diseases infecting different organs of human and describes the microorganisms involved in the process. The diseases caused by parasites and helminths are also included in the text since the infective stages of parasites as well as helminths are microscopic in nature and need to be distinguished from bacterial infections by appropriate diagnosis.