What makes us ill? It is the germs（细菌）. ________ are everywhere. The are very ________ . You can’t find them with your eyes, but you can see them ________ a microscope（显微镜）. There could be hundreds of them on a very small thing.
Germs are always found in ________ water. When we look at dirty water under the microscope, we shall see ________ . So your father and mother will not let you drink dirty water.
Germs are found not only in ________ . They are found ________ in air and dust（灰尘）. When you ________ your finger, if some dust from the floor goes into the cut, some germs would go into your finger. Then you will have much pain in it. Sometimes germs would go into all of your body, and you would have pain everywhere.
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