[Solving the mystery of cellular inclusions. A contribution to the history of virology]

In the middle of the 19th century cell inclusions were observed with increasing frequency in more and more diseases and were closely scrutinized by researchers working in different fields. Because of their distinct viewpoints, however, the various authors came inevitably to different conclusions. The morphologists interpreted the inclusions as artefacts or degenerative changes, the etiologists, on the other hand, took them for pathogenic protozoa, for cellular lesions inflicted by invisible agents or, conversely--for aggregated products of the cellular defense. Various morphological, parasitological and bacteriological methods have been used to clear up the pros and cons of these hypotheses. It was the rapid progress realized in virology at the middle of the 20th century that finally brought to light their real significance.