Most germs do not hurt people, and some even help us. Some tiny living things cause disease, and one early clue came from wine and milk, which seem far from medicine. Louis Pasteur showed that yeast, a living organism, causes alcoholic fermentation. He also showed that living organisms turn milk sugar into lactic acid. This pointed to tiny living things as the cause of change, including disease.

Pasteur kept testing his ideas through careful experiments. 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. He also built swan-neck flasks, which let air in but kept dust and airborne microbes out. Sterilized broth in these flasks stayed clear, which helped show that air alone cannot create life.

Two hospital clinics in Vienna can be compared. From 1840 through 1846, the number of mothers who died in the doctors' clinic was 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 the midwives did neither. In 1847 he had the students wash in chloride of lime first.

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. Some bacteria multiply quickly in the body and give off toxins, which are poisons that damage cells.

Robert Koch studied one disease and the bacterium behind it. He discovered the bacterium that causes tuberculosis in 1882. He won the 1905 Nobel Prize in Physiology or Medicine.

Edward Jenner used cowpox in 1796 to protect a boy from smallpox, which was an early vaccine. By the mid-1800s, scientists such as Pasteur were making vaccines from weakened versions of the germs that cause disease. A vaccine teaches your immune system to recognize a germ before the real germ arrives. It works by imitating an infection, so that the immune system gets to practice without the person getting the real illness.

Ilya Mechnikov studied the body's own defenders. In 1882 he proposed that some white blood cells work by capturing and destroying harmful bacteria, a process he called phagocytosis. He shared the 1908 Nobel Prize for work on immunity. These cells are called phagocytes, and they chew up invading organisms. One kind, the neutrophil, fights bacteria. Antibodies add a second kind of defense. They are proteins that lock onto specific germs.

In 1928 Alexander Fleming saw that a mold had grown on a plate of staphylococcus bacteria and that the bacteria had stopped growing in the area near the mold. He concluded that the mold made a substance that works against bacteria, and he named it penicillin. It became the basis for medicines that treat bacterial infections. Antibiotics are medicines that kill bacteria or stop them from growing. Taking them when they are not needed does not help and can cause side effects.

Even the simplest tool, handwashing, leaves a question open. Washing for about 15 to 30 seconds removes more germs than washing for less time, which is why many countries recommend about 20 seconds as of November 2026. But 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. Even so, handwashing lessons in communities have been measured to cut diarrhea by 23 to 40 percent.

Handwashing also matters for another reason, because it helps fight antibiotic resistance. Fewer infections means fewer antibiotics used, and overuse of antibiotics is the single most important factor behind resistance worldwide. Resistance is a natural process. Bacteria that survive a drug through genetic changes multiply and spread, so each use of an antibiotic selects for resistant survivors. It is the germs that become resistant to a medicine, not the person's body. That is why drugs should be used only when they are needed.

Words to know

• pasteurization: heat-killing germs
• fermentation: yeast changing sugar
• phagocytosis: cell swallowing germs
• penicillin: mold medicine
• resistance: drug immunity