Although improved sanitation in cities promoted a better quality of life and some improvements in health care, effective control of communicable disease required a great leap forward in medical knowledge and biological theory. Early reformers, including Chadwick, were seriously handicapped by their adherence to the prevailing miasmatic theory of disease — the belief that people contracted disease when they inhaled the bad odors of decay and putrefying excrement. In the 1840s and 1850s keen observation by doctors and public health officials pinpointed the role of bad drinking water in the transmission of disease and suggested that contagion was spread through physical contact with filth, not by its odors, thus weakening the miasmatic idea.

The breakthrough in understanding how bad drinking water and filth actually made people sick arrived when the French chemist Louis Pasteur developed the germ theory of disease. Pasteur (pas-TUHR) (1822–1895), who began studying fermentation for brewers in 1854, used a microscope to develop a simple test that brewers could use to monitor the fermentation process and avoid spoilage. He found that fermentation depended on the growth of living organisms and that the activity of these organisms could be suppressed by heating the beverage — a process that came to be called pasteurization, which he first implemented in the early 1860s. The breathtaking implication of this discovery was that specific diseases were caused by specific living organisms — germs — and that those organisms could be controlled.

By 1870 the work of Pasteur and others had demonstrated the general connection between germs and disease. When, in the middle of the 1870s, German country doctor Robert Koch (kawkh) and his coworkers developed pure cultures of harmful bacteria and described their life cycles, the dam broke. Over the next twenty years, researchers — mainly Germans — identified the organisms responsible for disease after disease. These discoveries led to the development of a number of effective vaccines, though some infections resisted treatment until scientists developed antibiotics in the middle of the next century.

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Acceptance of germ theory brought about dramatic improvements in the deadly environment of hospitals and operating rooms (see Chapter 18). In 1865, when Pasteur showed that the air was full of bacteria, English surgeon Joseph Lister (1827–1912) immediately grasped the connection between aerial bacteria and the problem of wound infection. He reasoned that a chemical disinfectant applied to a wound dressing would “destroy the life of the floating particles” (or germs). Lister’s antiseptic principle worked wonders. In the 1880s German surgeons developed the more sophisticated practice of sterilizing not only the wound but also everything — hands, instruments, clothing — that entered the operating room.

The achievements of the bacterial revolution coupled with the public health movement saved millions of lives, particularly after about 1880. Mortality rates began to decline dramatically in European countries (see Figure 22.1) as the awful death sentences of the past — diphtheria, typhoid, typhus, cholera, yellow fever — became vanishing diseases. City dwellers especially benefited from these developments. By 1910 a great silent revolution had occurred: the death rates for people of all ages in urban areas were generally no greater than those for people in rural areas, and sometimes they were lower.