The exact origins of the virus is a riddle scientists have been working hard to solve in a bid to halt its spread, especially in the lead-up to the annual hajj pilgrimage to Saudi Arabia in October.
Now an international team says blood tests were positive for antibodies in camels from Oman, meaning they had at some point been infected with Middle East Respiratory Syndrome Coronavirus (MERS-CoV), or a closely-related virus.
The findings suggest that Arabian or dromedary camels "may be one reservoir of the virus that is causing MERS in humans," said a statement that accompanied the study published in The Lancet Infectious Diseases journal.
MERS has killed 46 of the 94 people confirmed infected since September last year, according to the World Health Organisation.
Concerns about the virus, for which there is no vaccine, have led Saudi Arabia to restrict visas for the 2013 hajj, which sees millions of Muslims flock to the holy sites of Mecca and Medina every year.
Scientists had long suspected that like its cousin virus SARS, which killed hundreds of people in Asia 10 years ago, MERS may originate in bats.
It is unlikely, however, that these shy, nocturnal creatures are passing the virus on to humans, and the involvement of an intermediary "reservoir" animal is suspected -- with anecdotal evidence of patients having been in contact with camels or goats.
The virus is not very adept at jumping from person to person, though there have been isolated cases.
For the study, the team took blood from 50 camels from across Oman and another 105 in the Canary Islands, as well as llamas, alpacas, Bactrian camels, cattle, goats and sheep from the Netherlands, Chile and Spain.
They found MERS-like antibodies in all of the Omani camels and lower levels in 15 of those from the Canary Islands.
"What it means is that these camels some time ago have come across a virus that is very similar to MERS-CoV," the paper's senior author Marion Koopmans of the Netherlands' National Institute of Public Health and the Environment, told AFP.
According to the study, the Oman samples came from various locations in the country, "suggesting that MERS-CoV, or a very similar virus, is circulating widely in dromedary camels in the region."
But the team could not say when the animals had been exposed, or whether it was the exact same virus.
"For that, studies are needed that collect the right samples from camels while they are infected," said Koopmans. Other animals from the Middle East, like goats, must also be tested.
Dromedary camels are popular animals in the Middle East and North Africa, used for transport, meat and milk, as well as racing. There are an estimated 13 million of them in the world today -- all but a few domesticated.
A respiratory virus that causes fever and pneumonia, MERS has claimed lives in Jordan, Qatar, the United Arab Emirates, Saudi Arabia, France, Germany, Italy, the UK and Tunisia.
All people who had fallen ill outside the Arabian peninsula had either visited one of the Middle Eastern countries or had been infected by a person thought to have come from there.
"This looks like the big break that public health workers needed in the fight against the spread of MERS," University of Reading microbiologist Benjamin Neuman said of the study.
"This is the first hard evidence that camels may be the missing link in the chain of transmission."
The next step, he said, would be to look for the virus itself in camels and find out whether it is mutating in a way that makes it easier to infect humans.
Koopmans said the findings had by no means solved the puzzle, but was an important pointer for further research.
"Camels indeed are very important for the region, an important source of food, transportation and fun (racing), and we should certainly not jump to conclusions," she said.
"We need to find the virus first, and we need to know in more detail how people get infected. Only when that is clear, it may be possible to draw up some specific control measures."