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.

The breakthrough in understanding how bad drinking water and filth actually made people sick arrived when the French chemist Louis Pasteur (pas-TUHR) (1822–1895) developed the germ theory of disease. Pasteur’s experiments demonstrated 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 identified the organisms responsible for disease after disease. These discoveries led to the development of a number of effective vaccines.

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 benefited especially 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.