Most of theare caused by microorganisms like viruses, bacteria, fungi or parasites. Some harmless and even helpful microorganism lives in our body, but under certain circumstances they may cause . Some are communicable as it can be passed from one person to the other. Some are caused by food or water borne virus, and some are transmitted through insects or animal bites. Each and every infectious disease has its own individuality in signs and symptoms. Some of the general signs include flu, diarrhoea, coughing, fatigue and muscle aches. It is mandatory to consult a doctor if the condition goes worse.