Opinion: How a virus seemingly returned from the dead

(CNN)New York City has detected poliovirus in sewage, according to state and local health officials, suggesting likely local circulation of the virus. This comes as no surprise since last month a case of paralytic polio was confirmed in a resident in Rockland County -- just on the outskirts of the city. As the state health commissioner Dr. Mary T. Bassett said, "For every one case of paralytic polio identified, hundreds more may be undetected."