Look at your hands. You cannot see any germs on them, because germs are so tiny that you need a microscope to see them. Most germs do not hurt people, and some even help. Only some of them make people sick. Germs come in several kinds, including bacteria, viruses and fungi. Over the next pages, you will meet each kind and find out what your body does about them. Let us begin.

Let us start with bacteria. Bacteria are living things made of just one cell, and under a microscope they look like balls, rods or spirals. They live almost everywhere on Earth, including in and on your body. That may sound scary, but it is not, because some bacteria are helpful. The bacteria in your digestive system help your body digest food. So not every bacterium is bad news for you.

A virus is a different kind of germ. It is made of genetic material inside a coat of protein. A virus cannot make copies of itself alone. It has to get inside a living cell and use that cell to make copies. A cold is caused by a virus. Most colds go away on their own in about one to two weeks, so you do not have to stay sick for long.

What happens when a germ gets in? A germ makes you sick when it gets into your body and your body reacts. Some bacteria multiply quickly in the body and give off toxins, which are poisons that damage cells. So the bacteria are not just sitting there. They are busy making more of themselves, and making poisons as they go.

Germs spread when someone coughs or sneezes, and they also spread when you touch something that has germs on it. Germs can get into your body through your eyes, nose and mouth, so try not to touch your face with unwashed hands. When you sneeze or cough, cover your mouth and nose with a tissue. Then throw the tissue away. These small habits keep germs from moving from one person to the next.

Your body has a team to fight back, and it is called the immune system. It fights germs to keep you healthy. White blood cells are part of your body's defense, and they go after germs and fight them. Antibodies are proteins that lock onto specific germs. Each part of the team has its own job in the fight. Together, they help keep you well.

Here is a way to get ready before the real germ comes. A vaccine teaches your immune system to recognize a germ before the real germ arrives. Vaccines work by imitating an infection, so the immune system practices without the person getting the real illness. A vaccine can make you feel tired or sore for a day or two while your body learns. After that, your body is ready if the real germ ever comes.

Some medicines work on germs too. Antibiotics are medicines that kill bacteria or stop them from growing. They do not work on viruses, so they do not help a cold. Thick yellow or green mucus does not mean you need antibiotics for a cold. Take antibiotics exactly the way a doctor tells you to, and never share them with anyone else.

Now for the best tool you have. Washing your hands with soap and water helps keep germs from making you sick. Any soap works, because antibacterial soap is not better than plain soap. Scrub with soap for 20 seconds. That is about as long as humming Happy Birthday twice. Wash your hands before you eat and after you use the toilet. If you cannot use soap and water, a hand sanitizer with at least 60 percent alcohol can help. Try it the next time you wash up.

Words to know

• bacteria: tiny living germs
• virus: tiny germ
• vaccine: germ-training shot
• toxins: cell-harming poisons
• antibodies: germ-grabbing proteins