Polio used to be one of the most feared childhood diseases. It can attack the nervous system and - in just a few hours - leave someone paralysed.
If it paralyses the chest muscles, people are unable to breathe.
And the sarcophagus-like iron lung, which uses bellows to keep the patient breathing, was once a common sight on polio wards.
But all this feels like a long-forgotten era - and for good reason.
The disease has almost been wiped off the face of the planet.
The UK's last case of natural polio was in 1984.
And this is the context in which to consider the news polio has paralysed a young man in the US and, in London, a million children are being urgently immunised.
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The first case of wild polio in Africa for more than five years was reported in a three-year-old girl in Malawi.
It was the same strain found in Pakistan - although, nobody is sure how it travelled.
And in February, Malawi had to declare a wild polio outbreak.
Secondly, the vaccine used both in endemic countries and to deal with outbreaks can itself create a problem.
And this is what is now affecting the UK and other countries.
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The oral vaccine has been hugely successful - but this ability to revert to its more dangerous form is why countries aim to switch to an injection of dead virus once they are polio-free.
The UK has used injections since 2004.
What is now turning up in London sewers is virus that has come from using the oral vaccine elsewhere in the world.
Some of the samples show signs of regaining the ability to cause paralysis - and genetic analysis suggests the virus has been spreading.
It is also directly related to samples found in wastewater in the US and Israel.
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The progress we have made can be undone.
Scientific developments should make a difference - a new more stable version of the oral vaccine is less likely to revert back to causing paralysis, for example.
But ultimately polio needs to be tackled in the two remaining endemic countries - otherwise, the threat of outbreaks will always be there.
And that is a challenge not just about science or money but also of politics and society
The US was criticised for damaging efforts to defeat polio when it used a fake vaccine programme to try to find former al-Qaeda leader Osama Bin Laden.
We are so close to the end of polio - but we have been close for a long time.
The disease is a diminished threat - but it will not be over here until it is over everywhere.
https://www.bbc.co.uk/news/health-62492791