Polio has been eradicated in the UK in recent years after the deadly illness wreaked havoc in the 1950s.
But it has been discovered in several sewage samples collected from the London Beckton Sewage Treatment Works between February and May of this year.
Health officials in London have also started an emergency vaccination campaign among children as they try to prevent an outbreak of the virus.
So far no cases of polio paralysis have been detected, but what is the disease? And what happened last time there was a polio epidemic in the UK?
What is polio?
Polio is a life-threatening disease caused by the poliovirus.
Symptoms of polio include a high temperature, extreme fatigue, headaches, vomiting, neck stiffness and muscle pain.
In extreme cases, polio can cause paralysis, usually in the legs, although movement typically comes back within a few weeks or months.
But it can be life-threatening if it paralyses the muscles needed for breathing.
It mainly affects children under the age of 5, although it can also hit unvaccinated adults.
It has caused death or life-long paralysis in the past.
Where is polio found?
Polio is extremely rare now due to the success of the polio vaccine. It is primarily found in Afghanistan and Pakistan.
How does polio spread?
Polio spreads through bodily fluids. An infected person can spread it by coughing or sneezing. People can also catch it by coming into contact with the faeces of an infected person.
When were the last polio epidemics in the UK?
During the early 1950s the UK was rocked by polio epidemics, with as many as 8,000 people suffering paralytic poliomyelitis.
The epidemics ended with the introduction of the oral polio vaccine in 1962.
The last case of polio in the UK was in 1984 and it was was officially eradicated in the UK in 2003.
Are we vaccinated and how long does protection last?
The polio vaccine lasts 10 years. After this time, people need a booster vaccination.
The vaccine is injected into a person’s upper arm, with mild side effects reported afterwards.
What is an iron lung?
An iron lung is a respirator that stimulates breathing by enclosing a person’s body and varying the air pressure inside.
They are typically considered obsolete due to the invention of new breathing techniques and the eradication of polio.
Who is the man in the iron lung?
Paul Alexander contracted polio at the age of six in Texas in 1952. He was kept alive with the use of an iron lung.
Mr Alexander is still alive today at the age of 76 after living in an iron lung for 70 years.
Polio in Britain through the years - in pictures
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A group of mothers with their children wait outside Middlesex County Council Clinic for the first polio vaccinations in May 1956. All photos: Getty Images -
A polio vaccine being tested at the Glaxo Laboratories in Sefton Park, Buckinghamshire, in 1956. -
A child is injected with a vaccine against polio in 1956, the first stage of the biggest vaccination programme ever undertaken in Britain at the time. -
Four young children wait for injections against polio at the Fairfield Welfare Clinic in Wandsworth, south London, in 1956. -
Health minister Dennis Vosper examining the polio virus in 1957, during the production of Polivirin, Britain's anti-poliomyletis vaccine, at Glaxo's Virus Research Laboratories in Stoke Poges. -
A queue of factory workers waiting to receive polio vaccinations in one of the new London Mobile Immunisation Clinics at Highbury Grove in 1959. -
Junior members of staff at the Ministry of Health on Saville Row, London, line up to receive their inoculation against polio in 1959. -
Nurses in one of the many emergency polio vaccination clinics in Hull read the latest reports on the health scare in 1961.
