The poliovirus is transmitted from person to person by direct contact with infected oropharyngeal secretions or feces; the incubation period is 5 to 35 days. The virus enters through the alimentary tract, multiplies in the oropharynx and lower intestinal tract, and spreads to regional lymph nodes and the blood.

Complications from poliomyelitis include respiratory failure, pulmonary edema, pulmonary embolism, urinary tract infection, urolithiasis, atelectasis, pneumonia, cor pulmonale, soft-tissue and skeletal deformities, paralytic shock, and hypertension.

Inapparent (subclinical) infections constitute 95% of all poliovirus infections. Abortive or inapparent poliomyelitis (minor illness) accounts for 4% to 8% of all cases and causes slight fever, malaise, headache, sore throat, inflamed pharynx, and vomiting. The patient usually recovers within 72 hours. Most cases of abortive poliomyelitis go unnoticed.

Major poliomyelitis involves the CNS and takes two forms: nonparalytic and paralytic. Children commonly show a biphasic course in which the onset of major illness occurs after recovery from the minor illness stage. Nonparalytic poliomyelitis produces moderate fever, headache, vomiting, lethargy, irritability, and pains in the neck, back, arms, legs, and abdomen. It also causes muscle tenderness, weakness, and spasms in the extensors of the neck, back, hamstring, and other muscles during range-of-motion exercises. Nonparalytic polio usually lasts about a week, with meningeal irritation persisting 2 weeks. Paralytic poliomyelitis develops within 5 to 7 days of the onset of fever with symptoms similar to those of nonparalytic poliomyelitis: asymmetrical weakness of various muscles, loss of superficial and deep reflexes, paresthesia, positive Kernig’s and Brudzinski’s signs, hypersensitivity to touch, urine retention, constipation, and abdominal distention. The extent of paralysis depends on the level of the spinal cord lesions (cervical, thoracic, or lumbar).