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Most cases of C. difficile are not actually caused by the bug being spread round hospitals, a study suggests.

A team from the University of Oxford said "more and more deep cleaning ain't going to do any good".

in Oxfordshire for more than three years showed less than a fifth of cases had been spread between hospital patients.

Researchers said there was a growing awareness of animal and community sources of infection.

The gut bug is one of the most feared "hospital infections". It can be difficult to treat and deadly, especially in the elderly.

Rising levels of a particularly dangerous strain of the bacterium, alongside problems with the MRSA superbug, led to a deep-clean campaign across hospitals in the UK, and infection rates fell.

However, a study in the Oxford University Hospital NHS Trust area, between 2008 and 2011, showed that reducing cases even further may require a different approach.

Researchers essentially tried to build a family tree of the cases.

They took samples of the bacterium from every infected patient and looked at the gut bug's DNA.

If the genetic codes of bacteria in two patients are similar, it suggests that they came from the same source.

This genetic information was combined with patient records to try to work out where the infection was coming from.

Their conclusion, reported in the New England Journal of Medicine, was that just 18% of infections were being spread between patients in hospital.

One of the researchers, Prof Tim Peto, told the ´óÏó´«Ã½: "More and more deep cleaning ain't going to do any good."

He said we were probably being exposed to C. difficile all the time, but it became a problem only in vulnerable people.

"I think we're eating it all the time, probably from animals, and most of us get it and it doesn't matter."

Antibiotics play a crucial role in the infection. They create space in the gut for C. difficile to flourish by killing the other gut bacteria.

Prof Peto suggested that better use of antibiotics had helped reduce infection rates in the past decade and that more intelligent use of the drugs would be the best way to stop the infection in the future.

"Changing antibiotic policy is preventative, the focus is to stop someone becoming symptomatic," he said.

Prof Mark Wilcox, from the University of Leeds and Public Health England, said this was a "landmark study".

"The results have an important message for infection teams.

"Continuing on the same path to controlling C. diff will not ensure that all preventable cases are avoided, new measures are needed to prevent this bug spreading and being provoked to cause infection."

Commenting on the research, Prof Nigel Minton, from the Clostridia Research Group at the University of Nottingham, said: "Obviously hospital infection control measures have had a big impact on C. difficile cases.

"But there is a growing feeling that community-acquired C. diff is equally important and there are also studies suggesting possible transmission to humans from animals. This has quite clearly been demonstrated from pigs to humans in the Netherlands"

"Nursing homes are a major factor as well, it is where you get a lot of people susceptible to infection."