Infectious diseases are the disorder caused by pathogenic microorganisms, such as bacteria, viruses, parasites or fungi that spread from one person to another directly or indirectly. Such diseases are communicable or non-communicable. Many organisms live in and on our bodies are harmless or even helpful. Under certain circumstance, these organisms become a disorder due to some metabolic changes in them. Infections invade organism’s body tissues by disease-causing agents through their multiplication and the reaction of host tissues to the infectious agents and the toxins they produce.