 NPR's Gabriel Emmanuel reports that there are no known cases at this time, but a risk assessment is underway. The samples were positive for vaccine-derived polio, which is shed by individuals who've received the live oral vaccine. The origins in Gaza are still being investigated, but vaccine-derived polio is a growing problem and in severe cases can cause paralysis. It often spreads by an unvaccinated person coming into contact with water that's been contaminated by the virus.