Most germs do not hurt people, and some germs even help, so only some of them make people sick. For an infection to happen, germs must go through a chain of steps: they travel from a source, enter a body, invade tissues, multiply and cause a reaction. Germs spread when someone coughs or sneezes, and when you touch something that has germs on it. They can get into your body through your eyes, nose and mouth. A germ makes you sick when it gets into your body and your body reacts.

Germs come in several kinds, including bacteria, viruses and fungi. Bacteria are tiny 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. Some fungi are germs too, and a fungus causes athlete's foot. Parasites are living things that use other living things, like the human body, for food and a place to live.

A virus is a germ made of genetic material inside a coat of protein, and it works in a very different way from a bacterium. It cannot make copies of itself alone, so it has to get inside a living cell and use that cell to make copies. Often a virus ends up killing the cell it infected while making copies, which is how it damages the body. Only a small number of the many viruses on Earth can infect people.

Germs are very tiny, and you need a microscope to see them. Antonie van Leeuwenhoek of Delft was a cloth merchant who made microscopes with single powerful lenses, not the compound microscopes used today. His lenses magnified over 200 times, with clearer images than his colleagues could make. He was the first to see bacteria, and he also saw blood cells and tiny animals called rotifers and nematodes.

People are often told that Leeuwenhoek invented the microscope, but compound microscopes were invented around 1595, nearly forty years before he was born. His real gift was grinding lenses and describing carefully what he saw. It was that careful looking that opened the way to seeing the germs we study today.

Your body has defenses of its own. Skin is a barrier that blocks germs from getting into the body, and everyone is born with this general protection, called innate immunity. If germs get through, 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. Sneezing, coughing, diarrhea and a high fever come partly from the immune system fighting germs, not only from the germs themselves.

Vaccines let your body practice. A vaccine teaches your immune system to recognize a germ before the real germ arrives, and it works 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 you have met a germ, your immune system usually remembers it. Herd immunity adds another layer: when enough people are vaccinated, it is harder for a disease to spread to people who cannot have vaccines, such as those with weakened immune systems.

Soap works better than water alone because soap 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. Washing for about 15 to 30 seconds removes more germs than washing for less time. As of November 2026, many countries recommend about 20 seconds, which is about as long as humming a birthday song twice. Antibacterial soap is not better than plain soap. If you cannot use soap and water, a hand sanitizer with at least 60 percent alcohol can help.

Handwashing breaks the chain at the travel step, and the results have been measured. Handwashing education in communities has been measured to cut diarrhea by 23 to 40 percent. It has cut colds and other respiratory illness by 16 to 21 percent, and school absences from stomach illness by 29 to 57 percent. Washing your hands with soap and water helps keep germs from making you sick. Wash your hands before you eat and after you use the toilet.

Everyday habits stop germs before they travel. When you sneeze or cough, cover your mouth and nose with a tissue and then throw the tissue away. Try not to touch your face with unwashed hands, because that is one way for germs to reach your eyes, nose and mouth. Food can have germs that make people sick, and washing your hands before you eat or prepare food helps stop them.

In 1928 Alexander Fleming saw that a mold had grown on a plate of staphylococcus bacteria and that the bacteria had stopped growing near the mold. He concluded that the mold made a substance that works against bacteria. Antibiotics are medicines that kill bacteria or stop them from growing, but 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.

Taking antibiotics when they are not needed does not help and can cause side effects. It can also add to antibiotic resistance, which 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. So take antibiotics exactly the way a doctor tells you to, and never share them.

Words to know

• infection: germs causing harm
• multiply: make many more
• vaccinated: given a vaccine
• antibiotics: bacteria-killing medicines
• lenses: curved glass