The clearest evidence yet that Crohn's disease is caused by a type of bacteria blamed for a similar animal disorder has been published by US researchers.
Crohn's disease affects 100,000 people in Britain
Mycobacterium avium subspecies paratuberculosis (Map) was found for the first time in the blood of people with the disorder.
Half of the 28 people with Crohn's tested positive for Map.
Dr Saleh Naser, from the University of Central Florida, who led the research, said a large-scale study was needed.
Crohn's disease affects 100,000 Britons and can cause a variety of symptoms from a lack of appetite to chronic diarrhoea and abdominal pain.
The cause of the disease is not yet known but the Map bacterium, which causes Johne's disease, a similar intestinal disorder, in cattle, sheep and goats, was first linked to Crohn's 20 years ago.
Dr Naser's team took blood samples from 52 people - 28 with Crohn's, nine with ulcerative colitis, another inflammatory bowel disease (IBD), and 15 people without a IBD.
The live bacterium was found in the blood of 14 patients with Crohn's but in none of the people without the disorder.
It is the first time the bacterium has been found in the blood as other studies have focused on human tissue.
Two people with ulcerative colitis were also reported to have Map present but the study said this could have been because they had Crohn's.
Traces of Map DNA were found in some of the people with ulcerative colitis and three without an IBD.
The study suggested this was because Map is common in the environment with exposure most likely to happen through the food and water supply.
However, it is thought these people would not have developed Crohn's as people with the disorder have to be genetically susceptible first.
In the past, scientists have claimed Crohn's is passed to humans through milk.
Dr Naser said: "The fact that some people had Map DNA in their blood is worrying. It suggests the bacterium is not just opportunistic.
"It is present more than we think. We know cattle and sheep have this bacteria so it is possible animals may be spreading the disease and pasteurisation is not doing the job it should be."
He said the fact that half the people with Crohn's had Map was a "significant number" and future studies should result in a higher proportion.
"We only took a small blood sample, if we are right, Map will be present in a greater proportion of people with Crohn's.
"What we now need is more work to prove this theory."
Writing in the Lancet, Professor Warwick Selby, of the Gastroenterology and Liver Centre at the Royal Prince Alfred Hospital in Australia, agreed more research was needed.
"This report may still fall short of proving that Map is one of the causes of Crohn's disease but as with similar studies it raises many important questions.
"The findings now need to be replicated in other laboratories. Whatever one's view, Map cannot continue to be ignored in Crohn's disease."
Dr Martin Sarner, the honorary secretary of Core, formerly the Digestive Disorders Foundation, said if it could be proved Map caused Crohn's it would represent a huge breakthrough for patients.
"It would help in the treatment of the disease. It is terribly debilitating for people.
"At the moment people are with given medication, which just keeps the inflammation at bay or they have surgery which can involve cutting away tissue.
"And the problem they face is that it can just keep coming back."