A U.K. man has made medical history after it was discovered that a polio vaccine that he received as a child was never cleared from his body. In fact, it has been circulating in his gut for almost 30 years.

While it was known that some people could take extended periods of time to shed the virus from their system, this is by far the longest it has stayed in a person's body. The discovery has prompted researchers to issue a fresh warning of the virus, saying that new outbreaks could occur, especially in areas where people aren't sufficiently protected from the illness.

Researchers analyzed over 100 samples of the man's stool, finding mutated versions of the polio virus that were capable of causing paralysis. It is believed that the man, who has an immune deficiency, has been shedding the strain of polio for 28 years. He was originally given a full course of polio vaccinations, being given three weakened doses of the live virus at 5, 7 and 12 months of age. He was also given a booster shot when he was 7 years old.

The tests showed that the weakened version of the virus the man was given as a vaccine had mutated in his gut into a much more dangerous strain. Of course, those vaccinated against polio are not at risk of contracting the virus, but in countries where polio vaccinations are not routine, there is a risk of an outbreak occurring.

It's also important to note that the vaccination given to the man was through an older technique, the oral polio vaccine, which contained a weakened form of the virus, often given on a sugar cube. The U.K. switched to administering an inactivated polio vaccine in 2004. This vaccine is administered through an injection.

The new discovery was reported in the journal PLOS Pathogens. It could have serious implications for health officials who are battling the virus in the three countries that still have a circulation of polio. These countries are Pakistan, Afghanistan and Nigeria.