In 1943, 721 patients with poliomyelitis were admitted to the Contagious Disease Unit of the Los Angeles County General Hospital. Of these, 9.29 per cent (67 persons) came from families with multiple cases. There were 22 families with 2 cases each, 5 families with 3 cases and 2 families with 4 cases.
There were 38 secondary cases. Thirty-one (81.6 per cent) occurred within eight days or less of the initial case, and it is unlikely that any considerable number of the secondary cases were infected through exposure to the primary cases. Six (15.8 per cent) occurred between nine and fourteen days after the initial case and may or may not have been infected through exposure to the primary cases. One case (2.6 per cent) occurred after five weeks, which is a much longer period than is usually given as the upper limit for incubation.
In the previous five year period