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A previously unknown virus linked to obesity and diabetes has been discovered - and researchers say it lives in the gut of more than half the world's population.

Called crAssphage, it infects one of the most common types of gut bacteria, Bacteroidetes.

The researchers first noticed the virus in a computer simulation - then found it was present in more than half the people they tested.

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Called crAssphage, it infects one of the most common types of gut bacteria, Bacteroidetes (pictured).

The virus is as old as humans, it is believed. The researchers first noticed the virus in a computer simulation - then found it was present in more than half the people they tested.

The research appears today in Nature Communications, after Robert  Edwards, a bioinformatics professor at San Diego State University stumbled upon the discovery by accident.

The researchers were using results from previous studies on gut-inhabiting viruses to screen for new viruses.

In the DNA fecal samples from 12 different individuals, they 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 failed to find a match.

The researchers then screened for the virus across the database of the National Institute of Health's Human Microbiome Project (HMP), and Argonne National Laboratory's MG-RAST database, and again found it in abundance in samples derived from human feces.

To prove that the viral DNA they discovered in their computer data actually exists in nature, fellow SDSU virologist John Mokili used a technique known as DNA amplification to locate the virus in the original samples used to build NIH's database.

'So we have a biological proof that the virus they found with the computer actually exists in the samples,' Mokili said.

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

'It's not unusual to go looking for a novel virus and find one," Edwards said.

'But it's very unusual to find one that so many people have in common.

'The fact that it's flown under the radar for so long is very strange.'

The discovery could lead to a new understanding of obesity

The fact that it's so widespread indicates that it probably isn't a particularly young virus, either.

'We've basically found it in every population we've looked at,' Edwards said.

'As far as we can tell, it's as old as humans are.'

He and his team named the virus crAssphage, after the cross-assembly software program used to discover it.

Bacteriodetes bacteria live toward the end of the intestinal tract, and they are suspected to play a major role in the link between gut bacteria and obesity.

What role crAssphage plays in this process will be a target of future research.

Further details about crAssphage have been difficult to come by.

It's unknown how the virus is transmitted, but the fact that it was not found in very young infants' fecal samples suggests that it is not passed along maternally, but acquired during childhood.

The makeup of the viral DNA suggests that it's circular in structure.

Further laboratory work has confirmed that the viral DNA is a singular entity, but it's proven difficult to isolate.

'We know it's there, but we can't capture it quite yet,' Edwards said.

Once the virus is isolated, he hopes to delve into its role in obesity.

The virus might also be used to prevent or mitigate other diseases affected by the gut such as diabetes and gastroenterological maladies.

Once these processes are better understood, Edwards envisions one day the possibility of personalized medicine based on this virus.

'This could be a key to personalized phage medicine,' he said.

'In individuals, we could isolate your particular strain of the virus, manipulate it to target harmful bacteria, then give it back to you.'