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Although polio paralysis is the most visible sign of polio infection, fewer than 1% of polio infections ever result in paralysis. Most cases (90%) produce very mild or no symptoms and usually go unrecognized. A further 5% to 10% of polio infections result in aseptic meningitis, a viral inflammation of the outer covering (meninges) of the brain. The rest involve mild flu-like symptoms common to other viral infections - mild fever, sore throat, abdominal pain, and vomiting. No one knows why only a small percentage of infections lead to paralysis. Several key risk factors have been identified as increasing the likelihood of paralysis in a person infected with polio. These include:
- immune deficiency
- pregnancy
- removal of the tonsils (tonsillectomy)
- intramuscular injections
- strenuous exercise
- injury
THE HIDDEN VIRUSPoliovirus can spread widely without even being 'seen'. As most people infected with poliovirus have no signs of illness, they are never aware they have been infected.After initial infection with poliovirus, the virus is shed intermittently in faeces (excrement) for several weeks. During that time, polio can spread rapidly through the community.
HOW IS POLIO SPREAD?Poliovirus is spread through person-to-person, fecal-oral contact. Where hygiene and sanitation are poor, young children are especially at risk. Young children who are not yet toilet-trained are a ready source of transmission, regardless of their environment. Polio can be spread when food or drink is contaminated by faeces. There is also evidence that flies can passively transfer poliovirus from faeces to food.The disease circulates "silently" at first, and may infect hundreds of people, depending on the level of sanitation, before the first case of polio paralysis emerges. Because of this silent transmission and the rapid spread of the disease, WHO considers a single confirmed case of polio paralysis to be evidence of an epidemic - particularly in countries where very few cases occur.
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