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In ancient times disease was believed to be caused be supernatural forces, poisonous vapors called miasmas, and imbalances among the four humors thought to be present in the body. The role of the four humors (blood, phlegm, yellow bile [choler], and black bile in disease had been widely accepted since the time of the Greek physician Galen (129–199). However, Girolamo Fracastoro and a few others had suggested that invisible organisms produced disease support for the idea that microorganisms cause disease. This is under wise known as the germ theory of disease which gradually became accepted in the 19th century.

Agostino Bassi (1773–1856) first identified that a microorganism could cause disease when he demonstrated in 1835 that a silkworm disease was due to a fungal infection. He also suggested that many diseases were due to microbial infections. In 1845 M. J. Berkeley (1803–1889) proved that the great potato blight of Ireland was caused by a water mold, and in 1853 Heinrich de Bary (1831–1888) showed that smut and rust fungi caused cereal crop diseases. Following his successes with the study of fermentation, Pasteur was instructed by the French government to investigate the pèbrine disease of silkworms that was disrupting the silk industry. After several years of work, he showed that the disease was due to a protozoan parasite.

Further evidence for the germ theory of disease came from the work of the English surgeon Joseph Lister (1827–1912) on the prevention of wound infections. Lister, impressed with Pasteur’s studies on the involvement of microorganisms in fermentation and putrefaction, developed a system of antiseptic surgery with the aim to prevent microorganisms from entering wounds. Instruments were heat sterilized, and phenol was used on surgical dressings and at times sprayed over the surgical area. The approach was remarkably successful and transformed surgery.  It also provided strong indirect evidence for the role of microorganisms in disease because phenol, which kills bacteria, also prevented wound infections.


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