Chronic fatigue syndrome is real, say scientists from New York's Cornell University who have discovered bacteria found in the gut may be a factor.

Chronic fatigue syndrome is a condition that causes extreme and lasting fatigue, and many have argued it is a psychological condition.

However, a new study, published in the Microbiome journal, supports growing evidence it is a physical illness.

Researchers at New York's Cornell University discovered that a large proportion of people with chronic fatigue syndrome (CFS) had higher levels of a inflammatory marker in their blood caused by bacteria in the gut.

Molecular biologist and lead author Maureen Hanson says discovering the bacteria blueprint is important because it offers new clues to why people have symptoms commonly associated with CFS.

"There's a biological difference between people with CFS and healthy people," she said.

Scientists from Cornell screened the blood and stools of 48 people with CFS and 39 without the condition for a variety of bacteria.

They found 83 per cent of those with CFS had higher levels of lipopolysaccharides, large molecules found in bacteria, in their blood.

That indicated bacteria moved from the gut into the bloodstream, where they could produce various symptoms of disease, according to the authors of the study.

Prof Hanson says the long-lasting idea that CFS is a psychological illness should be abandoned.

"Our work demonstrates that the gut bacterial microbiome in chronic fatigue syndrome patients isn't normal, perhaps leading to gastrointestinal and inflammatory symptoms in victims of the disease," she said.