One of the earliest ideas about germs came from the ancient Greek historian Thucydides. He believed that during the plague of Athens, the infection spread from one diseased person to another.

The ancient Roman poet Lucretius added to this with his own theory. He said there were “seeds” in the world that could make people ill. He was influenced by the idea that contagious diseases weren’t spread by direct touch, but instead by airborne spore-like seeds.

In the 11th century, the Muslim scientist Ibn Sina theorized that diseases can be spread by breathing, water, and dirt. He also came up with the idea of a quarantine, or self-isolation, to prevent people from spreading illnesses.

For centuries, people believed in the miasma theory—the idea that bad smells and dirty air cause diseases. People thought that if they breathed in stinky air, they would get sick. The miasma theory was popular up until the 19th century.

People also used to believe in spontaneous generation, the idea that living things (and germs) could just appear from nothing! This made people think that diseases caused germs, and not the other way around.

A French chemist named
Louis Pasteur set out to disprove spontaneous generation. In the 1850s, Pasteur was given a task by a winemaker to discover why their wine kept going bad. Using a microscope, he found bacteria in the wine, which he thought was the reason why the wine always spoiled.

To test this theory, Pasteur experimented with a swan-neck flask. He poured broth into two swan-neck flasks and boiled the liquid, killing the microbes. He then broke the neck off one of the flasks, which made the liquid go bad, but the flask that wasn’t broken didn't go bad. This proved that microbes didn’t just appear from nothing; they were in the air.

The German physician Robert Koch further developed Pasteur’s theory by identifying which bacteria cause diseases. He did this by growing bacteria using agar jelly and a stain to dye the bacteria so he could see them under the microscope.

Koch discovered the bacterium that causes anthrax in 1876, the bacterium that causes tuberculosis in 1882, and the bacterium that causes cholera in 1883.

Germs are too small to be seen by the human eye and can only be viewed under a microscope. There are many different types of germs. Some are rod-shaped (bacilli), spiral-shaped, or even sphere-shaped (cocci).