The human body is protected by millions of fighting units, circulating in the blood stream. These consists of different types of soldiers, each group having its own specific function.
Central control can order out new units when disease invades the body. During times of peace the numbers are reduced and the fighters become patrols.A healthful diet, physical fitness and positive emotional states can stimulate and strengthen the body's immune system. On the other hand, illness, drugs and excessive stress can weaken it.