In this classic international bestseller, first published in 1926, bacteriologist and pathologist Paul de Kruif gives an accessible and fascinating account of the first scientists to explore the microscopic world. Included are the father of microbiology, Antonie van Leeuwenhoek, who created lenses to study the microscopic world, and Louis Pasteur, whose discovery of a vaccine for rabies would save many lives. The legacy of the thirteen researchers and scientists profiled continues today, influencing research in the current fight against viruses such as SARS-COVID and HIV-AIDS.