The pioneering work of Louis Pasteur, Rudolf Virchow, Robert Koch, and others firmly established that germs cause disease. This knowledge made it easier to track the spread of disease because now, at least, scientists knew what to look for, even if they did not know exactly where to look. Nevertheless, many challenges lay ahead. More powerful microscopes came into use during the late 1800s, allowing researchers to study germs in increasingly greater detail. Experiments involving germs were conducted in labs in Germany, Great Britain, the United States, Italy, France, and other countries.

The ultimate goal of the scientists and doctors who performed these experiments was to find effective cures for various diseases. But these researchers knew that attaining that goal would be difficult and would take considerable time. Finding out how each disease attacks the body would unquestionably be a...