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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 Virus

Poliovirus can spread widely without even being 'seen'. Most people
infected with poliovirus (at least 90% of all cases) have no signs of illness and 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 feces. 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. 
 

(Text and Photos Courtesy of the World Health Organization)