Until the 1930s most scientists believed that viruses were small bacteria, but following the invention of the electron microscope in 1931 they were shown to be completely different, to a degree that not all scientists were convinced they were anything other than accumulations of toxic proteins.
1930-ndateni uskus enamik teadlasi, et viirused on väiksed bakterid, aga elektronmikroskoobi leiutamise järel 1931. aastal ilmnes, et viirused on midagi täiesti teistsugust, mistõttu hulk teadlasi arvas, et need on vaid mürgised valkude kogumid.