Germs are so small that you need a microscope to see even one. Most of them do no harm, and some even help you. Only some germs make people sick. Seeing something that small took special tools, such as the single-lens microscopes made by a cloth merchant.

Antonie van Leeuwenhoek lived in Delft, in Holland. He was a cloth merchant with no university degree. He made simple microscopes that used a single powerful lens, not the compound microscopes used today. His lenses magnified more than 200 times and gave clearer images than his colleagues could make. With them he became one of the most important discoverers in the history of biology.

Germs come in several kinds, including bacteria, viruses and fungi. Bacteria are tiny living things made of just one cell. Under a microscope they look like balls, rods or spirals. A virus is made of genetic material inside a coat of protein, and a fungus can cause athlete's foot. Parasites are living things that use other living things, like the human body, for food and a place to live. They range from one-celled protozoa to worms you can see.

Not every germ is an enemy. Bacteria live almost everywhere on Earth, including in and on your body. Some are helpful, because bacteria in your digestive system help your body digest food. Only a small number of the many viruses on Earth can infect people.

A germ makes you sick when it gets into your body and your body reacts. A virus cannot make copies of itself alone, so it has to get inside a living cell and use that cell to make copies. Some bacteria work another way. They multiply quickly in the body and give off toxins, which are poisons that damage cells. Either way, germs take energy from the body and can damage healthy cells.

Sneezing, coughing, diarrhea and a high fever come partly from the immune system fighting germs, not only from the germs themselves. A fever is a body temperature of 100.4 degrees Fahrenheit (38 degrees Celsius) or higher. Most fevers are caused by infections. A fever helps the immune system kick into action against the germs. So a feverish day can be a sign that your defenses are working, even though it feels bad while it lasts.

Your body has layers of defense against invaders. Skin is a barrier that blocks germs from getting into the body. Everyone is born with this general protection, which is called innate immunity. If germs get past it, white blood cells go after them. Some, called phagocytes, chew up invading organisms, and one kind, the neutrophil, fights bacteria. Antibodies are proteins that lock onto specific germs. After you have met a germ, your immune system usually remembers it, and your antibodies stay in your body in case that germ comes back. A vaccine teaches your immune system to recognize a germ before the real germ arrives.

Colds show how germs get from person to person. More than 200 viruses can cause a cold, and in the United States rhinoviruses cause the most. Colds spread through tiny droplets in the air and on things people touch. Adults average two to three colds a year, and children often get more. Most colds go away on their own in about one to two weeks.

Antibiotics are medicines that kill bacteria or stop them from growing. Antibiotics 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.

Soap works better than water alone because it lifts dirt and microbes off the skin, and people also scrub longer when they use soap. The temperature of the water does not seem to change how many germs a wash removes. Antibacterial soap is not better than plain soap. A good wash means scrubbing with soap for 20 seconds, about as long as humming a birthday song twice. Germs can get into your body through your eyes, nose and mouth, so try not to touch your face with unwashed hands. If you cannot use soap and water, a hand sanitizer with at least 60 percent alcohol can help.

Why does washing after the toilet matter so much? A single gram of human poop, about the weight of a paper clip, can hold one trillion germs, which is why this habit matters. Wash your hands before you eat and after you use the toilet. You cannot see these germs without a microscope.

Words to know

• microscope: viewing tool
• bacteria: tiny living things
• virus: tiny germ
• parasites: living freeloaders
• antibodies: germ fighters
• toxins: poisons