'Silent' Polio Cases Could Reignite Epidemics, Study Warns

Only a few hundred cases of paralytic polio still exist worldwide, thanks to massive vaccination campaigns.  But a new study concludes that so-called silent cases of polio can infect populations for years, threatening to reignite disease epidemics.   The World Health Organization designates a region polio-free if there have been no active infections for three or more years. However, people can be infected with the polio virus for years and not develop any symptoms. It's...

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