Germs are so tiny that you need a microscope to see them. Many people think that all germs are bad, but that is not true. Most germs do not hurt people, and some even help. Only some of them make people sick. They come in several kinds, including bacteria, viruses and fungi, and each kind works in its own way.

Bacteria are tiny living things made of just one cell. Under a microscope they look like balls, rods or spirals. They live almost everywhere on Earth, including in and on your body. Many are helpful. The bacteria in your digestive system help your body digest food. 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. There are many viruses on Earth, but only a small number of them can infect people. A germ makes you sick when it gets into your body and your body reacts. It can get in through your eyes, nose and mouth.

Once inside, they take energy from the body and can damage healthy cells. Some bacteria multiply quickly in the body and give off toxins, which are poisons that damage cells. Germs spread when someone coughs or sneezes, and when you touch something that has germs on it. This is how one sick person can pass a germ to another.

A cold is caused by a virus, and it spreads through tiny droplets in the air and on things people touch. Adults average two to three colds a year, and children often get more. That means that catching a cold is very common. Most colds go away on their own in about one to two weeks, while your body does the work of getting better.

Your body fights back with its immune system. White blood cells go after germs and fight them. Antibodies are proteins that lock onto specific germs. A fever is a body temperature of 100.4 degrees Fahrenheit (38 degrees Celsius) or higher. After you have met a germ, your immune system usually remembers it. Your antibodies stay in your body in case that germ comes back.

Vaccines use that memory. 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. That is a small cost for a lot of practice.

Antibiotics are medicines that kill bacteria or stop them from growing. They do not work on viruses, so they do not help a cold, and thick yellow or green mucus does not mean you need them. Take antibiotics exactly the way a doctor tells you to, and never share them. Over time, a problem can grow. Antibiotic resistance happens when bacteria change so that an antibiotic no longer kills them. They keep growing. It is the germs that become resistant to a medicine, not the person's body.

Your best protection is simple. Scrub with soap and water for 20 seconds, which is about as long as humming Happy Birthday twice. Antibacterial soap is not better than plain soap. Any soap works. Wash before you eat and after you use the toilet, and 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. Small habits like these stop tiny germs from traveling to the next person.

Words to know

• bacteria: tiny living germs
• antibodies: germ-locking proteins
• resistance: medicine-proof change
• toxins: cell-harming poisons
• vaccine: germ-training shot