Germs are very tiny, and you need a microscope to see them. Bacteria are tiny living things made of just one cell, and under a microscope they look like balls, rods or spirals. Parasites are living things that use other living things, like the human body, for food and a place to live, and they range from one-celled protozoa to worms you can see. A virus is a germ made of genetic material inside a coat of protein. Most germs do not hurt people, and some germs even help, so only some of them make people sick.

One large effort to study the microbes that live with us was the NIH Human Microbiome Project, which examined 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 human microbiome includes both helpful and potentially harmful microbes, and most are helpful partners, although some, in smaller numbers, can cause disease. Even the basic count has changed. As of November 2026, the record shows that NIH once described microbial cells in the body as outnumbering human cells by about ten to one, while a 2016 study re-counted and estimated about one microbial cell for each human cell.

A germ makes you sick when it gets into your body and your body reacts. When germs get inside the body they take energy from it and can damage healthy cells, and some bacteria multiply quickly in the body and give off toxins, which are poisons that damage cells. Viruses work in a different way, because a virus cannot make copies of itself alone. It has to get inside a living cell and use that cell to make copies, and often a virus ends up killing the cell it infected while making copies, which is how it damages the body.

Antonie van Leeuwenhoek of Delft, Holland, was a cloth merchant with no university degree, yet he made simple microscopes and became one of the most important discoverers in the history of biology. His microscopes were single powerful lenses, not the compound microscopes used today, and his lenses magnified over 200 times, with clearer images than his colleagues could make. He is often called the inventor of the microscope, but compound microscopes were invented around 1595, nearly forty years before he was born. His gift was grinding lenses and describing carefully what he saw. He was the first to see bacteria, and he also saw blood cells and tiny animals called rotifers and nematodes.

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

Pasteur also built swan-neck flasks, which let air in but kept dust and airborne microbes out. 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. The process is called pasteurization, and it 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 discovered the bacterium that causes tuberculosis, and he won the 1905 Nobel Prize in Physiology or Medicine. Koch's postulates are four criteria for deciding whether a particular bacterium causes a particular disease, and as of 2025 many researchers still use them as a guide. Science corrects itself, too. Koch did not believe human and animal tuberculosis were connected, but the Nobel Foundation notes he was not entirely correct.

The 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. Some white blood cells, called phagocytes, chew up invading organisms, and one kind, the neutrophil, fights bacteria. In 1882 Ilya Mechnikov proposed that some white blood cells work by capturing and destroying harmful bacteria. He called the process phagocytosis, and he shared the 1908 Nobel Prize for work on immunity. 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.

Some of what you feel when you are sick comes from your own defenses. Most fevers are caused by infections, and a fever helps the immune system kick into action against the germs. A fever is a body temperature of 100.4 degrees Fahrenheit (38 degrees Celsius) or higher. Sneezing, coughing, diarrhea and a high fever come partly from the immune system fighting germs, not only from the germs themselves.

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, compared with 36.2 per 1,000 in the midwives' clinic. 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, about the same as the 13.3 per 1,000 in the midwives' clinic. How do we know the clinics were really different? Patients went to the two clinics on alternate days, which by accident made a controlled trial, and with so many births, 42,795 from 1840 to 1846, chance could confidently be ruled out. No one knew about germs in 1847, so Semmelweis could not explain why handwashing worked, and his idea had little effect at the time. Antisepsis became routine in childbirth care only in the 1880s, after bacteria were discovered.

For an infection to happen, germs must 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, and colds spread through tiny droplets in the air and on things people touch. A cold is caused by a virus, and more than 200 viruses can cause one. In the United States, rhinoviruses cause the most colds. Germs can get into your body through your eyes, nose and mouth, so try not to touch your face with unwashed hands. A single gram of human poop, about the weight of a paper clip, can hold one trillion germs, which is why washing after the toilet matters.

Handwashing breaks the chain at the travel step. Handwashing education in communities has been measured to cut diarrhea by 23 to 40 percent, colds and other respiratory illness by 16 to 21 percent, and school absences from stomach illness by 29 to 57 percent. 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. Handwashing 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, and only a small share of microbes cause illness, so as of November 2026 the best washing time is still not settled.

Edward Jenner used cowpox in 1796 to protect a boy from smallpox, an early vaccine. By the mid-1800s scientists such as Pasteur were making vaccines from weakened versions of disease-causing germs. 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. 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.

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, and it 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 and add to antibiotic resistance.

Antibiotic 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, so each use of an antibiotic selects for resistant survivors, and resistant germs can 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, and it also estimated that bacterial resistance was associated with more than 4.7 million deaths worldwide in 2021. Both are estimates as of November 2026, and handwashing helps fight resistance, because fewer infections means fewer antibiotics used, and overuse of antibiotics is the single most important factor behind resistance worldwide.

Words to know

• postulates: required criteria
• controlled: carefully compared
• antisepsis: germ prevention
• phagocytosis: cell swallowing germs
• resistance: drug immunity