Germs are very tiny, so you need a microscope to see them. Each of us carries a crowd of them, and this is the story of how people learned what they do. Germs come in several kinds, including bacteria, viruses and fungi, and bacteria are tiny living things made of just one cell. Most germs do not hurt people, and some even help. Only some of them make people sick. A germ makes you sick when it gets into your body and your body reacts.

The NIH Human Microbiome Project studied the microbes that live in and on the human body and their roles in health and disease. Before it began, this community was largely unstudied. The microbiome includes both helpful and potentially harmful microbes. Most are helpful partners, while some, in smaller numbers, can cause disease. As of November 2026, here is where the count stands. NIH once described microbial cells in the body as outnumbering human cells by about ten to one, but a 2016 study re-counted and estimated about one microbial cell for each human cell.

Louis Pasteur showed that yeast, a living organism, causes alcoholic fermentation, and that living organisms turn milk sugar into lactic acid. This pointed to tiny living things as causes of change, including disease. Pasteur was not the first to say that germs cause disease. His experiments gave the idea strong support against the older miasma theory, which held that bad air caused disease.

Pasteur's swan-neck flasks let air in but kept dust and airborne microbes out, and sterilized broth in them stayed clear, which helped show that air alone cannot create life. In 1864 he found that heating wine to 50 to 60 degrees Celsius (122 to 140 degrees Fahrenheit) killed the microbes that spoiled it. This process, called pasteurization, later became common for milk.

In 1876 Robert Koch proved that one bacterium, Bacillus anthracis, causes anthrax, and he was the first to prove that a specific bacterium causes a specific disease. In 1882 he found the bacterium that causes tuberculosis, and in 1905 he won the Nobel Prize in Physiology or Medicine.

Also in 1882, Ilya Mechnikov proposed that some white blood cells work by capturing and destroying harmful bacteria, a process he called phagocytosis. White blood cells are part of your body's defense, and they go after germs and fight them. Some, called phagocytes, chew up invading organisms, and one kind, the neutrophil, fights bacteria.

In the 1840s two hospital clinics in Vienna had very different results. From 1840 through 1846, deaths of mothers in the doctors' clinic were 98.4 per 1,000 births, while in the midwives' clinic they were 36.2 per 1,000 births. Ignaz Semmelweis noticed that medical students went from handling bodies to examining mothers without washing their hands, while midwives did neither. In 1847 he had students wash in chloride of lime first.

After handwashing began, deaths in the doctors' clinic fell in 1848 to 12.7 per 1,000 births, which was about the same as the 13.3 per 1,000 in the midwives' clinic. Today we know why the change worked. For an infection to happen, germs must travel from a source, enter a body, invade tissues, multiply and cause a reaction, and washing hands breaks the chain at the travel step.

Lessons on hand washing in communities have been measured to cut diarrhea by 23 to 40 percent, colds and other breathing 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. Germs can get into your body through your eyes, nose and mouth, so try not to touch your face with unwashed hands.

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. Washing for about 15 to 30 seconds removes more germs than washing for less time, and as of November 2026, many countries recommend about 20 seconds. Most 20-second studies counted how many microbes were removed, not how many people stayed healthy. Only a small share of microbes cause illness, so as of November 2026 the best washing time is still not settled. If you cannot use soap and water, a hand sanitizer with at least 60 percent alcohol can help.

Your body has an immune system that fights germs to keep you healthy. A vaccine teaches your immune system to recognize a germ before the real germ arrives. Edward Jenner used cowpox in 1796 to protect a boy from smallpox, an early vaccine, and by the mid-1800s scientists such as Pasteur were making vaccines from weakened versions of disease-causing germs. Vaccines work by imitating an infection, so the immune system practices without the person getting the real illness. When enough people are vaccinated, it is harder for a disease to spread to people who cannot have vaccines. After you have met a germ, your immune system usually remembers it.

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 the mold made a substance that works against bacteria, and he named it penicillin, which became the basis for medicines that treat bacterial infections. 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. Taking antibiotics when they are not needed does not help and can cause side effects.

Overuse of antibiotics adds to antibiotic resistance. Resistance happens when bacteria change so that an antibiotic no longer kills them, and they keep growing. It is the germs that become resistant to a medicine, not the person's body. Bacteria that survive a drug through genetic changes multiply and spread, and each use of an antibiotic selects for resistant survivors. Resistant germs can also pass their resistance genes to other germs that were never exposed to the drug. The World Health Organization estimated that about 1 in 6 lab-confirmed bacterial infections worldwide were resistant to antibiotics in 2023, an estimate as of November 2026. Washing hands helps fight resistance, because fewer infections means fewer antibiotics used. Take antibiotics exactly the way a doctor tells you to, and never share them.

Words to know

• microbiome: body's microbe community
• miasma: bad air
• anthrax: bacterial disease
• pasteurization: heat treatment
• resistance: drug immunity