Last week, public health officials declared a ‘national incident’ after they found vaccine-derived poliovirus in London sewage samples. No cases of polio symptoms have been reported but there is evidence the virus is spreading. So what does it mean to have found the virus almost 20 years after the UK was declared polio-free? Ian Sample speaks to epidemiologist Nicholas Grassly to find out how worried we should be and what it means for the global effort to eradicate polio.. Help support our independent journalism at theguardian.com/sciencepod