II.

Symptoms

The virus usually enters the body through the alimentary tract and spreads along nerve cells to affect various parts of the central nervous system. The incubation period ranges from about 4 to 35 days. Early symptoms include fatigue, headache, fever, vomiting, constipation, stiffness of the neck, or, less commonly, diarrhoea and pain in the extremities. Because nerve cells that control muscular movement are not replaced once they are destroyed, poliovirus infection can cause permanent paralysis. When nerve cells in respiratory centres, which control breathing, are destroyed, the victim must be kept alive by an iron lung (a means of artificial respiration). For every paralytic case of poliomyelitis, however, there may be 100 nonparalytic cases.