People infected with a deadly virus that emerged in Saudi Arabia last year may have caught it from one-humped camels.
Scientists studying what kind of animal "reservoir" may be fuelling the outbreak in humans have found strong evidence it is widespread among dromedary camels in the Middle East.
The Middle East Respiratory Syndrome Coronavirus (MERS-CoV) can cause coughing, fever and pneumonia.
It has been reported in people in the Gulf, France, Germany, Italy, Tunisia and Britain.
The World Health Organisation (WHO) says 46 people have died out of a total 94 confirmed cases, the majority in Saudi Arabia.
The camels are used in the region for meat, milk, transport and racing.
Experts not involved in the study hailed its findings as a major step towards solving the mystery of, and ultimately controlling, the MERS virus.
Days after identifying the new virus in September in a Qatari patient at a London hospital, British scientists had sequenced part of its genome, mapped out its "phylogenetic tree" and found it was related to a virus found in bats.
Further work by scientists at Germany's University of Bonn suggested it may have come through an intermediary animal after they conducted a detailed case study of a male patient from Qatar who said he owned a camel and a goat farm.
Benjamin Neuman, a microbiologist at Britain's University of Reading, said the findings look like the big break that public health workers need in the fight against the virus.
He said: "The biggest mysteries ... have been how people are becoming infected with a virus of bats, and why it is happening in the Middle East.
"By showing that one-humped camels have a history of MERS-like infections, these scientists may have helped answer both questions at once."
The Dutch-led team, whose study was published in The Lancet Infectious Diseases journal, gathered 349 blood serum samples from a variety of livestock animals, including dromedary camels, cows, sheep, goats, and some animals related to dromedaries.
The animals were from several different countries, including Oman, the Netherlands, Spain and Chile.
While no MERS-CoV antibodies were found in blood serum taken from 160 cattle, sheep, and goats from the Netherlands and Spain, they were found in all 50 samples from camels in Oman.
The Oman samples came from different areas, suggesting that MERS-CoV, or a very similar virus, is circulating widely in dromedary camels in the region, the researchers said.