Why has polio been found in London, New York and Jerusalem, and how dangerous is it?

Vaccine-derived polio was detected in wastewater in the British capital, London, and in New York in the United States, with one case of paralysis reported in New York state.

Polio, a deadly disease that used to paralyze tens of thousands of children every year, is spreading in London, New York and Jerusalem for the first time in decades, spurring catch-up vaccination campaigns.