Decrease Told In Polio CasesA decrease of almost 50 per cent In reported cases of poliomyelitis in Illinois so far this year, compared with the corresponding period of 1955, is shown in a statement issued by the state Department of Public Health, In contrast, only a slight decrease is noted in the national incidence pf the disease.For the first 18 weeks of, this year, Illinois had 24 of the 1,463 cases reported nationally. In xl955,. Illinois reported 46 cases for the similar period, compared to 1,599 for the entire country.'Hie use of protective vaccine may certainly be regarded as one of several possible factors which have brought, about this favorable Illinois showing, according to Dr. Boland R. Cross, state health .-director.