Germ Theory of Disease

The germ theory of disease evolved in the latter half of the 19th century. This fundamentally new understanding of infectious diseases became accepted with vital results for the health of humankind. The theory is anchored in the independent research of Louis Pasteur (1861) and Robert Koch (1862–63), although bacteria were discovered in 1675.

Louis Pasteur found microbes to be behind the fermentation of sugar into alcohol and the souring of milk and developed a heat treatment (pasteurization) that killed microorganisms in milk, which then no longer transmitted tuberculosis or typhoid. Pasteur also developed new vaccines against such infections as anthrax and rabies. His work on fermentation from the 1860s became familiar to Joseph Lister, who introduced antiseptic techniques in hospitals with the crucial positive result of decreasing the spread of infection and resultant mortality.

Another great success in bacteriology and human disease in the late 19th century was achieved by Robert Koch thanks to his implacable belief that germs cause diseases and to his almost superhuman tenacity. In two consecutive years (1882–83), he identified the bacteria that cause tuberculosis and cholera, respectively. Koch and his students opened the door for the golden age of microbiology. Four fundamental postulates about microorganisms were published in 1890; they are partially true from the perspective of later knowledge.

There were many famous scientists and social activists who did not accept germ theory, initially, or at all. Rudolf Virchow (1821–1902) denied its validity for most of his life. Florence Nightingale ridiculed the idea of germs until her death in 1910. However, in 1886, the theory entered the leading American pediatric textbook by Job Lewis Smith (1827–97) in which Koch's research on the tubercle bacillus was named the most brilliant discovery of the last decade.

Toward the end of the 19th century, the growing world of microbes included discoveries by [Page 318]Dmitri Ivanowski (1892), and Martinus Beijerinck (1898), who revealed tiny infectious agents (“filtrable viruses”) that were too small to be seen with the conventional microscope and could pass through bacteria-stopping filters. In 1928, Sir Alexander Fleming discovered penicillin.

Louis Pasteur found that microorganisms in wine were responsible for spoilage and developed a unique heating technique to prevent the process. He applied his findings to raw milk, which stopped the transmission of tuberculosis and typhoid.

Both the miasma and germ theories of disease causation stimulated an expansion of the policy of increasing personal, surgical, and public hygiene in Europe and the United States in the late 19th to early 20th centuries. Germ theory's preventive health innovation included protective inoculations.