It may be that hundreds of people have contracted polio in the current outbreak, but do not realise.
The polio virus has been detected in wastewater in both London and New York.
Polio is a disease it was thought was almost eradicated outside of Pakistan and Afghanistan.
The disease was declared beaten in the U.S. in 1979.
A consistent vaccine programme has meant that most people are immune to it.
However, a few cases have sprung up around the world: in Ukraine, Israel and now there is a case of paralysis from polio near New York City.
It is thought that the virus may be once again spreading among people who are not vaccinated.
How does polio spread?
Polio is very contagious and spreads from person to person and through contaminated water — usually via faecal particles.
It can also spread by coughs and sneezes, but that is less likely.
What are the symptoms of polio?
Most people have no symptoms of polio infection, which is what can make it difficult to track and contain.
It may be that hundreds of people have contracted polio in the current outbreak, but do not realise.
Around one-quarter of people experience a few days of flu-like symptoms, including fever, headache, sore throat and nausea.
A small number of people, though, will get more serious symptoms.
Polio is most dangerous to children under five-years-old.
The virus can enter the spinal chord, causing paralysis and the possibility of permanent disability and death.
Vaccination
Most people in wealthy nations are routinely vaccinated against polio.
In the U.S. around 93 percent of 2-year-olds have had the polio vaccine, according to the Centers for Disease Control and Prevention.
For those who are vaccinated, there is no need to do anything.
Research has shown that people retain protective antibodies in their blood for decades after vaccination.
For those who are unvaccinated, some health officials are recommending polio shots.
Unvaccinated people are at a higher risk of more serious side-effects, such as paralysis.
Health agencies have begun offering polio shots, especially for young children who are not already vaccinated.
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