Germ theory of disease defines that the invasion of microorganisms such as bacteria, viruses, fungi, or protists in our body causes certain types of diseases. These microorganisms are too small to be seen with our naked eye and require a magnifying microscope for better visualization. They grow and reproduce within the host body and cause infectious diseases. Not only pathogenic organisms cause severe disease but also environmental, genetic factors, and potential host influence the severity of the disease.
Early theories of disease
- In ancient Greece, people thought that the infectious seeds present in the air or in food causes disease but not by direct contact with an infected person.
- This concept was later revised by Girolamo Fracastoro in the middle ages that the disease could be caused even by direct or indirect contact and via long distances as well.
- The disease-causing seed remains for a long time which was further confirmed and on the basis of length of dormancy of seed, diseases were categorized.
- The spontaneous generation theory by Francesco Redi was proved wrong later. In his experiment, his finding was the maggots were only found on the covered surface.
- Anton van Leeuwenhoek later refuted the spontaneous theory and confirmed the presence of microorganisms that caused the infection.
- The microorganisms causing the disease were only visualized on a microscope which was later postulated by Richard Bradley.
- The French chemist and microbiologist Louis Pasteur proved that the fermentation and putrefaction are caused by an organism found in the air while the German physician Robert Koch identified tuberculosis and cholera causing organisms.
- Robert Koch established the scientific statement that each specific disease is caused by a specific microorganism and demonstrated it with an experiment with anthrax isolated from a diseased host.
This demonstrated experiment with anthrax to the diseased host is known as Koch’s postulate which consists of four rules:
- The microorganisms must be in all individuals who is affected by the disease.
- The microorganisms must be isolated and grown in a culture media from the individual with the disease.
- The cultured organism when subjected to a healthy individual must cause disease.
- The microorganism must be re-isolated from the diseased host and must be similar to that of the original microorganism from the previous culture.
Limitations of Koch’s postulate
Koch’s postulates have some inherent limitations in some fact while some of the postulates could be fulfilled.
- Viruses and some particular bacteria such as leprosy causing bacteria cannot be cultured in the laboratory this meant that not all Koch’s postulates are fulfilled.
- There is also an unavailable animal model for the trail infection of leprosy disease.
- The third Koch’s postulate describes that the infectious organism must cause disease which is not mandatory because asymptomatic carriers, immune response, and genetic resistance are also possible.
- Harmless bacteria are also capable of causing disease if an individual is an immunocompromised patient or the bacteria might contain some extra virulence factor making it pathogenic.
Miasma theory was a theory described in the 19th century before germ theory. Until then miasma theory described that the disease-causing agent originates from the pollution or decaying matter producing foul air or that contaminates drinking water and air. In ancient Greek, miasma was known as pollution or bad air that was identifiable by a foul smell that causes diseases which was the product of environmental factors like contaminated water, foul air, and poor hygienic condition.
Experiment of Louis Pasteur
Louis Pasteur was the first to demonstrate the experiment of disease that was caused by microorganisms, not the air in the environment itself which was described before in the miasma theory.
In his experiment, he first used freshly boiled broth and a flask under the following conditions:
- Pasteur took a freshly boiled broth in a long curved goose necked flask. The bent neck of the flask prevented the particles from falling into the broth yet still allowing the air to flow freely.
- For a long period of time, the flask was free of microorganism contamination.
- He then exposed the boiled broth with a filter to prevent particles in which showed no growth of organisms and even in the vessel with no filter with the air passing freely, no growth was observed.
- Lastly, he broke the long neck of the flask making it wide open with the freshly boiled broth where the microorganism growth was observed which arrived from the exposed environment as spores on dust.
This experiment disproved the theory of spontaneous generation which described that the microorganisms are generated within the broth or the air itself rather it was from the particulates in the air. This was the last and most important theory experimenting on the germ theory of diseases.