Free The Great Influenza by John M Barry Page A

prominent and highly regarded British epidemiologist continued to advocate the miasma theory, and after the 1918 influenza pandemic, climatic conditions were scrutinized in a search for correlations.
    The 'filth' theory of disease was almost a corollary of the miasma theory. It also suited Victorian mores perfectly. Fear of 'swamp gas' (often a euphemism for the smells of fecal matter) and installation of indoor toilets were all part of the Victorian drive to improve sanitation and simultaneously to separate the human body from anything Victorians found distasteful. And filth often is associated with disease: lice carry typhus; contaminated water spreads typhoid and cholera; rats through their fleas spread plague.
    Both the miasma and filth theories had sophisticated adherents, including public health officials and some extremely gifted scientists, but the most scientific rival of the germ theory explained disease in terms purely of chemistry. It saw disease as a chemical process. This theory had much to recommend it.
    Not only had scientists used chemistry as a lens that brought much of biology into focus, but some chemical reactions seemed to mimic the actions of disease. For example, advocates of the chemical theory of disease argued that fire was a chemical process and a single match could set off a chain reaction that ignited an entire forest or city. They hypothesized that chemicals they called 'zymes' acted like a match. A zyme started a series of chemical reactions in the body that could launch the equivalent of fermentation (infection.) The chemical theory of disease, without the name, has in fact largely been validated. Scientists have clearly demonstrated that chemicals, radiation, and environmental factors can cause disease, although usually only through long-term or massive exposure and not, as the zymote theory hypothesized, by suddenly igniting a cascade of reactions.)
    Ultimately this theory evolved to suggest that zymes could reproduce in the body; thus they acted as both catalysts and living organisms. In fact, this more sophisticated version of the zymote theory essentially describes what is today called a virus.
    Yet these theories left many scientists unsatisfied. Disease often seemed to germinate, grow, and spread. Did there not then have to be a point of origin, a seed? Jacob Henle in his 1840 essay 'On Miasmata and Contagia' first formulated the modern germ theory; he also offered evidence for the theory and laid out criteria that, if met, would prove it.
    Then, in 1860, Pasteur proved that living organisms, not a chemical chain reaction, caused fermentation, winning converts to the germ theory. The most important early convert was Joseph Lister, who immediately applied these findings to surgery, instituting antiseptic conditions in the operating room and slashing the percentage of patients who died from infections after surgery.
    But the work of Robert Koch was most compelling. Koch himself was compelling. The son of an engineer, brilliant enough to teach himself to read at age five, he studied under Henle, was offered research posts, but became a clinician to support his family. He did not, however, stop investigating nature. Working alone, he conducted a series of experiments that met the most rigid tests and discovered the complete life cycle of the anthrax bacillus, showing that it formed spores that could lie dormant in the soil for years. In 1876 he walked into the laboratory of Ferdinand Cohn, one of Welch's mentors, and presented his findings. They brought him instant fame.
    He subsequently laid down what came to be known as 'Koch's postulates,' although Henle had earlier proposed much the same thing. The postulates state that before a microorganism can be said to cause a given disease, first, investigators had to find the germ in every case of the disease; second, they had to isolate the germ in pure culture; third, they had to inoculate a susceptible animal with the germ and the animal then