 He suggested taking the so-called abortive and nonparalytic cases as seriously as the grave ones with paralysis, since they were – as he emphasized – instrumental in the spread of the disease.He assumed that the agent could be passed on by presumably healthy persons, and he was the first to find that polio was not exclusively, not even mainly, a disease affecting the central nervous system.Based on his observations he came to the conclusion that the incubation period of polio was three to four days, which had long been disputed but was confirmed in the middle of the twentieth century.