All around us there are invisible forms of life which we call germs. They are in the air, in the soil, in the water we drink and the food we eat. Many of them are harmless or even beneficial to man, but others may cause diseases. The human body has many natural weapons to fight off the attack of the harmful germs. For instance the digestive juices and the blood itself kill of many kinds of germs. But certain ones enter the body and start an infection. Then the antigerm “soldiers” in the body spring into action. These are the white corpuscles in the blood. They can pass right through the thin walls of blood vessels and they can wander all over the body. The white blood corpuscles gather at the point of attack and destroy the germs by feeding on them.