Germ theory of disease

The Germ theory of disease is a theory in biology. It says that small organisms (called germs), also known as microbes, cause some diseases. These diseases are called infectious diseases. The germ theory states that small organisms cause a reaction in the body of those who are infected. The body's reaction to infection is called a disease.

Many scientists and doctors in history figured out that diseases are caused by microscopic organisms. Even after the microscope was invented, people still didn't know that germs caused diseases. People long believed that "bad air" from stinky trash dumps and from rotting meat was the cause of diseases. So people thought that covering their mouth and nose with a cloth would help filter out the "bad air." Scientists and doctors would use garlic and perfumes to ward off the bad air. But people still got sick and even died, so this theory was wrong. Polluted air is not the cause of disease.

One problem with the old theories of disease was that people believed that living things were spontaneously generated. Spontaneous generation is when something like a fly grows from a small speck of smelly meat. That old theory is called abiogenesis. In the seventeenth century, Francesco Redi (February 18, 1626 – March 1, 1697) discovered that flies lay eggs that become maggots. Prior to this, people thought that maggots came from rotting meat. He discovered this by sealing some meat in a jar and watching it. No maggots were found on the sealed meat. He also put some meat in a jar and covered it with gauze. Maggots were found on the gauze but not inside the jar. But when he placed meat in an open jar, maggots were found on the meat and inside the jar. Experiments like this proved that maggots come from flies who lay eggs, not from rotting meat. Later scientists would prove that diseases did not come from the air. Diseases are spread by infection.