When infection occurs in a person, it may transmit to other parts of his or her body, increasing the severity of the disease. It may spread directly via tissues as happens in bad cold, through the blood as in blood poisoning, by the nerves as in rabies. These are the four ways by which the disease may spread. Although the characteristics of various infectious diseases generally differ from each other. There are certain well marked stages which are common to all of them. After the entry of the germs in human body, there are three distinct stages, as given below:-

• Incubation period:- This is the interval from the time the germs enter the body and the appearance of actual symptoms. The length of this period differs in case of different diseases. Normally the period is from one to many days. During this, the germs settle down, multiply and produce their poison. But, the patient does not communicate the disease to others.

• Active demonstrable period:- At this stage, characteristics symptoms appear and the patient is definitely sick. Generally the duration of most of the diseases is short, but in case of tuberculosis, the symptoms may be present for years. During this time, the patient notices headache, fever, etc.

• Convalescent period:-  This period follows the active period, when symptoms are pronounced. During this period, recovery process is at work and the health of the patient begins to get restored. But at this stage, there is a likelihood of complications, involving other organs, either by spreading of the same germs from other germs. So we should always guard against such complication.