Biologists have discovered an extremely widespread virus that could be as old as humans and could play a major role in obesity and diabetes.

More than half the world's population is host to the newly described virus, named crAssphage, which infects one of the most common types of gut bacteria, Bacteroidetes, the findings showed.

This phylum of bacteria is thought to be connected with obesity, diabetes and other gut-related diseases.

"It is not unusual to go looking for a novel virus and find one. But it is very unusual to find one that so many people have in common. The fact that it has flown under the radar for so long is very strange," said Robert Edwards, a bioinformatics professor at the San Diego State University in the US.

In the DNA fecal samples from 12 different individuals, the researchers noticed a particular cluster of viral DNA, about 97,000 base pairs long, that the samples all had in common.

When Edwards and his colleagues checked this discovery against a comprehensive listing of known viruses, they came up empty.

This was a new virus that about half the sampled people had in their bodies that nobody knew about.

The fact that it is so widespread indicates that it probably is not a particularly young virus, either.

"We have basically found it in every population we have looked at," Edwards said.

"As far as we can tell, it is as old as humans are," he said.

The study appeared in the journal Nature Communications.