The near total eradication of polio worldwide is one of the most hard-won achievements of global public health in the last century.
That's why it is a major blow that traces of the virus have been detected in London.
Sure, the risk to children and young people in the UK is low. Children are routinely vaccinated against the virus so it will have little chance of spreading widely.
Even if it does spread among the minority of people who have not been vaccinated, a rigorous public health response should hopefully be able to bring it under control quickly.
But the fact polio can even get a toehold in a country with a sophisticated healthcare system and good levels of vaccine coverage is a stark reminder that polio has the potential to make a comeback until it's eradicated completely.
It also raises urgent questions about just how, and why, the UK is seeing evidence of polio transmission at all. No Western European country has seen an outbreak of vaccine derived polio virus (VDPV) in at least a decade.
Over the last few decades, a delicate balancing act has been struck in phasing out the oral polio vaccine "OPV," which despite being the most effective tool for ending polio outbreaks, also carries the risk of reigniting them.
It works using a live, but weakened form of the virus. If vaccine coverage is not high enough, that weakened virus can survive long enough to mutate and revert to its potentially deadly form.
This is what we now know has occurred in London. The good news of course, our public health system was able to spot it.
OPV was phased out in the UK more than 15 years ago once wild polio virus had been eradicated. It was replaced by inactivated polio vaccine (IPV). A similar phased retreat has been going on globally with OPV now only being routinely used in the two remaining endemic countries, Pakistan and Afghanistan, also in northern Nigeria where regular outbreaks have been occurring.
The main public health effort has rightly been focused on those countries.
Meanwhile, uptake of the polio vaccine used to keep the virus at bay here, has been slowly declining - particularly in London where suspected transmission of the virus is happening.
Polio can cause irreversible paralysis in around 1 in 200 children infected. Of them, 5 to 10% die if their respiratory system is paralysed. Only older people in the UK still live with the injuries caused by polio when they were children, or can remember the devastating impact it once had on families.
After so much progress in stamping polio out, it would be a very great shame if its dangers were forgotten now.