During the 1798 century the first virus was discovered. Therefore, Edward Jenner developed and introduced the first vaccine against smallpox.
Furthermore, physicians and scientists shared observations and results amongst each other, and made the decision to create The Germ Theory. Physicians such as William Harvey was steadfast that Miasma was provoked by bad air imbalance. Additionally, Robert Hooke and Anton van Leeuwenhoek had developed microscopes to observe microorganisms. After these discoveries physicians knew there bacteria and germs in the air that could cause sick diseases.
During 1861, Louis Pasteur proved that spontaneous generation (production of living organisms from, non-living matter). Moreover, Pasteur did more tests and studies to upgrade the use of this vaccine.
Initially, during the 1880's the Miasma Theory was developed. The Miasma theory was poisonous air made up of decomposing material that was the cause of diseases. Many of the people were sickened by Cholera, scarlet fever, tuberculosis, and referred to as Black Death. Moreover, many people died due to this bad air imbalance.
Over the centuries, in the 1900's Alexander Fleming discovered Penicillin and was able to originate a treatment for Penicillin.
After many centuries, physicians and scientists are confident that diseases are generated by microorganisms, bacteria, viruses, protozoans, and fungi. However, diseases these current days aren't as deadly and people can survive for long periods of time. Diseases can be improved by lifestyle habits such as; a diet and exercising.