GERM THEORY OF DISEASE.
Germ theory of disease was postulated to show that microorganisms are the causes of infectious
diseases.
But prior to the discovery of microorganisms in the 17th century, many theories were circulated about
the origin of disease. For example, the ancient Greeks proposed the miasma theory, which held that
disease originated from particles (bad air/smell) emanating from decomposing matter such as that in
sewage or cesspits. Such particles infected humans in close proximity to the rotten material. Diseases
including the Black Death, which ravage Europe’s population during the Middle Ages were thought
to have originated in this way.
Other forms of germ theory were proposed in the late Middle Ages by physicians including Ibn Sina
in 1025 and Girolamo Fracastoro in 1546 who proposed in his essay De Contagione et Contagiosis
Morbis, that seed-like spores may be transferred between individuals through direct contact,
exposure to contaminated clothing or through air. However, such views were held in disdain in
Europe where miasma theory remained dominant among scientists and doctors, and they would be
largely forgotten until the 19th century
In 1847, Hungarian Obstetrician, Ignas Semmelweis observed that mothers who gave birth in
hospital ward staffed by physicians & medical students were more likely to suffer and die from
Puerperal Fever after childbirth (10%-20% mortality rate) than were mothers in ward staffed by
midwifes (1% mortality rate). Semmelweis observed medical students performing autopsies and then
subsequently carrying out vaginal examination s on living patients without washing their hands in
between. He suspected that that the students carried disease from the autopsies to the patients they
examined. His suspicions were supported by the untimely death of a friend, a physician who
contracted a fatal wound after postmortem of a woman who had died of a puerperal infection. The
dead physicians wound had been caused by a scalpel used during the examination, and his
subsequent illness & death closely paralleled that of the dead patient.
Although Semmelweis did not know the true cause of Puerperal fever, he proposed that physicians
were somehow transferring the causative agent to their patients. He suggested that the number of
puerperal fever cases could be reduced if physicians and medical students simply washed their hands
with chlorinated lime water before and after examining every patient. When this practice was
implemented, the maternal mortality rate in mothers cared for by physicians dropped to same 1%
mortality rate observed among mothers cared for by midwives. Despite the great success, many
discounted Semmelweis’s work at the time, and physicians were slow to adopt the simple procedure
of handwashing to prevent infections in their patients because it contradicted established norms for
that period.
Around the time Semmelweis was promoting hand washing in 1948, British physician John Snow
conducted studies to track the source of cholera outbreaks in London by tracking the outbreak to two
specific water sources, both of which were contaminated by sewage. Snow ultimately demonstrated
that cholera bacteria were transmitted via drinking water. The work of both Semmelweis and Snow
clearly refuted the prevailing miasma theory of the day, showing that disease is not only transmitted
through the air but also through contaminated items. The theory of Semmelweis, Snow and other
physician was disputed by other scientist because with their theories, the cause of disease was not
fully understood.
In 1856, while studying the cause of beer & wine spoilage (i.e., why wine turn sour), Louis Pasteur
discovered that microorganism were responsible for the spoilage through the process of
fermentation. He called attention to the importance of microorganisms in everyday life and stirred
scientists to think that if bacteria could make the wine ‘sick’, then perhaps they could cause illness in
human. His works encourage the belief that microorganisms were in the air and could cause diseases.
This was the foundation for the germ theory of disease because it was this time the role of microbes
in causing infectious disease began to be slowly accepted.
It was Robert Koch who eventually develop a way to prove that specific microbe caused a particular
disease. He was able to prove that the cause of a specific disease could be attributed to a specific
microbe. Robert Koch and his colleague were able to definitely identify the causative pathogens of
specific diseases, including anthrax, tuberculosis & cholera.
He established four (4) criteria to identify the causative agent of a particular disease. This includes
i)
ii)
iii)
iv)
The microorganism or other pathogen must be present in all cases of the disease
The pathogen can be isolated from the disease host and grown in a pure culture in the lab.
The pathogen from the pure culture must cause the disease when inoculated into a healthy
susceptible laboratory animal
The pathogen must be re-isolated from the new host and shown to be the same as the
originally inoculated pathogen.
Below are the steps involved in Koch’s experiment.
i)
Microorganisms are isolated from a diseased or dead animal
ii)
The microorganisms are grown in pure culture
iii)
Microorganisms are identified
iv)
Microorganisms are injected/inoculated into a healthy lab. animal
v)
Disease is reproduced in the lab. animal
vi)
The microorganisms are isolated from the animal and grown in a pure culture
vii)
Microorganisms are identified.
At the end of the experiment, Koch’s postulate that “a specific infectious disease is caused by a
specific microbe”.