He began his scientific career as an assistant in the Pathological Anatomy laboratory from Budapest (1874–1881). In 1885 he was appointed professor of histopathology at the Faculty of Medicine in Budapest. The same year, he discovered a parasitic sporozoan of the ticks, named "Babesia" in his honor (of the family "Babesiidae"), and which causes a rare and severe disease called babesiosis. Later that year, he publishes the first treatise of bacteriology in the world, "Bacteria and their role in pathological anatomy and histology of infectious diseases", which he co-authored with Cornil.