An infectious disease is one, which can be transmitted from one person to another or, sometimes, to or from animals. All Infectious diseases are usually caused by microbes. Most microbes are essential components of our Environment and do not cause diseases. Those that cause diseases are called pathogenic organisms (pathogens). pathogens that are solely responsible for the transmission of diseases are mostly bacteria, viruses, protozoa and helminths and a disease is transmitted by transfer of these organisms from one person’s body to another.