Wilhelm was born to dressmaker Christoph and Caroline Dönitz and even as a young boy showed an interest in natural history and illustration. He studied medicine from 1859 to 1864 at the University of Berlin. He continued to work as a lecturer and researched on aspects of comparative zoology, anatomy and histology under Karl Bogislaus Reichert leading to being appointed Professor in 1873. He also worked with Friedrich Theodor von Frerichs. At the same time he was offered the Chair of Anatomy in Tokyo. He left Germany on 10 July 1873 for Japan after marrying Martha Schirmeister. In 1877 Japan was in a civil war and with the end of the Tokugawa dynasty, the new Meiji government was interested in German medicine especially to deal with matters of sanitation and public health. During vacations he studied Myriapoda, Arachnida (Arachnida) and Mollusca (Bivalvia) and collected these in Hakone, Odawara and Fuji. He produced all the illustrations accompanying his descriptions. In 1886 he returned to Berlin and worked with Robert Koch at the Hygiene Institute and later at the Institut für Infektionskrankheiten (Institute for Infectious Diseases) founded in 1891. He became director of the Bacteriology Laboratory at Bonn where he studied cholera. From 1896 he worked at the Institut for Serumforschung und Therapie at Steglitz under Paul Ehrlich. He then moved to become director of the Institute for Infectious Diseases where he acted as director during the years when Robert Koch was travelling.
