There was evidence of genetic susceptibility to poliomyelitis, but in the mid 1930's that evidence and the idea of genetic susceptibility disappeared from the literature and the collective research psyche. Alternative hypotheses, not amenable to test, were adopted but seldom formulated. Subsequent evidence for genetic susceptibility was ignored. The ill-fated vaccine trials in 1935 presented a psychological watershed for researchers. The later vaccines owed their success to unwritten and untested hypotheses: genetic susceptibility might have been an insuperable barrier to testing vaccines in children. Now that eradication is probable, new research suggests that many people may be susceptible to paralysis and that plans must be made for possible reappearance of the disease.