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.