Have you ever been ill? When you are ill, you must be unhappy because your body becomes hot, and there are pains all over your body. You don't want to work, you stay in bed, feeling very sad.
     What makes us ill? It is germs (细菌). Germs are everywhere. They are very small and you can't find them with your eyes, but you can see them with a microscope. They are very very  small and there could be hundreds of them on a very small thing.
    Germs are always found in dirty water. When we look at dirty water under the microscope, we shall see them in it. So your father and mother will not let you drink dirty water.
    Germs aren't found only in water. They are found in air and dust. If you cut your finger, if some of the dust from the floor goes into the cut (割开处), some of the germs would go into your finger. Your finger would become big and red, and you will have much pain in it. Sometimes the germs would go into all of your body, and you would have pain everywhere.