Overview

At the beginning of the twentieth century the germ theory of disease was alive and well. However, it would take time and skill to identify the specific causes of many of the infectious diseases. Robert Koch (1843-1910) and Louis Pasteur (1822-1896) had identified bacteria as the causative agents in many diseases, and scientists could stain these organisms and identify their shapes under the microscope.

Important areas developed early in the twentieth century that added to knowledge about pathogenic (disease-causing) organisms. Filterable viruses, organisms so small that they passed through a filter, were discovered in the 1890s. These viruses were so small and mysterious that many unexplained diseases were attributed to them. Also, one-celled animals, or protozoa, and a group of parasitic worms emerged as new suspects.

In 1901 the mysterious African sleeping...