You cannot see germs with your eyes alone. They are so tiny that you need a microscope to see even one. Most germs do not hurt people at all. Some germs even help you. Only some germs make people sick.

One kind of germ is called bacteria. Each one is a tiny living thing made of just one cell. Under a microscope, bacteria look like balls, rods or spirals. They live almost everywhere on Earth, even in and on your body. Some are good for you. The ones in your gut help you digest food.

A virus is another kind of germ. It is made of genetic material inside a coat of protein. A cold is caused by a virus. Colds spread in tiny drops in the air and on things people touch. Most colds go away on their own. That takes about one to two weeks.

Germs spread when someone coughs or sneezes. They also spread when you touch something that has them on it. They can get into your body through your eyes, nose and mouth. So try not to touch your face with hands that are not clean.

Your body has an immune system. It fights germs to keep you healthy. White blood cells are part of this defense. They go after germs and fight them. Sometimes your body gets a fever. A fever is a body temperature of 100.4 degrees Fahrenheit (38 degrees Celsius) or higher.

A vaccine helps your immune system get ready. It teaches your immune system to recognize a germ before the real germ arrives. Then your body already knows how to fight when that germ shows up.

You can help your body, too. Wash your hands with soap and water. Scrub for 20 seconds, about as long as humming Happy Birthday twice. Wash before you eat and after you use the toilet. This helps keep germs from making you sick.

When you sneeze or cough, cover your mouth and nose with a tissue. Then throw the tissue away. Germs spread when someone coughs or sneezes, so a covered sneeze keeps them from moving to the next person.

Words to know

• bacteria: tiny living germs
• virus: tiny germ
• vaccine: germ-training shot
• fever: hot body