Symptoms and diagnosis:

Polio is an interesting disease in that 90-95% of those who become infected remain asymptomatic, allowing the virus to circulate silently throughout a community before an apparent case of paralytic polio is seen.

If the poliovirus never extends beyond the small intestine, it does no apparent damage. However, it is when the poliovirus enters the bloodstream that it targets nerve cells and begins to do serious damage.

The virus replicates in the lymph tissue of the pharynx and extends to the central nervous system. The virus targets the motorneurons of the anterior horn cells of the spinal cord and brainstem. It is its multiplication and lytic infection of these neurons that leads to apoptosis of the affected neuron. These nerve cells cannot be regenerated, and the affected muscles no longer function.

The muscles of the lower extremities are affected more often than the upper extremities. The limbs become lifeless and floppy. In extreme cases, paralysis of the thorax and abdomen can lead to quadriplegia.

In the most severe cases, the poliovirus attacks the motor neurons of the brain stem, inhibiting the ability to break, speak, and swallow. This form of paralytic polio is referred to as bulbar poliomyelitis.

Polio mainly affects younger children, but the disease can have increasingly drastic effects as age increases. Several factors have been identified to explain why such a small percentage of injections lead to paralysis, including immune deficiency, pregnancy, tonsillectomy, injury, strenuous exercise, and intramuscular injections.

For the remaining 5%, most suffer minor fever and malaise, and only about 1% suffer a major aseptic meningitis phase where the meninges of the brain becomes inflamed. The individual may suffer headache, vomiting, fever, and stiffness of the back or legs. These symptoms typically last from ten to two days and are followed by a complete recovery.

Less than 1% of polio patients, about one in two hundred infections, result in paralysis, but the risk of permanent damage made polio one of the most dreaded childhood diseases of the 20th century.

This paralysis can range from a single muscle to the entire body, and in extreme cases where respiration is impaired, the use of artificial ventilation is required to improve the chance of survival. 5 to 10% of polio patients die from the immobilization of respiratory muscles.

Of the patients who develop paralytic symptoms, about 25% sustain severe, permanent disability, another 25% recover with only mild disabilities, and 50% recover completely.