The Polish microbiologist and philosopher of science, Ludwik Fleck (1896-1961), was a pioneer in constructivist history and philosophy of science. Based on studies in the history of syphilis, Fleck hypothesized that many established scientific facts are linked, in their development, to pre-scientific "proto-ideas." In 1935, Fleck proposed that the history of germ theories could be approached through his thesis on proto-ideas. His proposal, however, remained little more than a vague suggestion and was never developed in further detail. This paper introduces the concept of proto-ideas and discusses the central epistemological and historiographical implications of Fleck's thesis. The Fleckian approach offers an attractive alternative to positivist reconstructions of the early history of germ theories and provides a useful framework for a deeper understanding of the sociocultural background of the development of modern knowledge of infection.