Many camels in Saudi Arabia have been infected with the virus that causes Middle East respiratory syndrome (MERS), but the virus rarely spreads from the animals to people, a new study says. 

The MERS virus first appeared in 2012 and causes a respiratory illness that has killed 30 percent of the people infected with it, but it's not well understood.

In the new study, researchers tested blood from 45 people who were exposed to camels in Saudi Arabia, including 12 people who had direct contact with a herd of dromedary (one hump) camels while some of the animals were infected with MERS. These 12 participants had repeated contact with the infected camels for more than a month. For instance, they fed and groomed the camels and, in some cases, drank milk from the infected animals. The researchers also analyzed blood from 146 people who lived in the same region but who did not work with camels.

None of the people in the study had antibodies against MERS in their blood, meaning they likely had not been infected with MERS in the past. [8 Things You Should Know About MERS]

The researchers concluded that the MERS virus "was not highly transmissible from dromedaries to humans with various levels of exposure to this infected dromedary herd."      

Still, the researchers stressed that, although rare, it's still possible for camels to transmit MERS to humans. It may be more common for the virus to spread from camels to people "in other settings in which humans are exposed over sustained periods to animals among which virus prevalence is higher," the researchers wrote in their findings.

In fact, a study published last June provided strong evidence that a 44-year-old man from Saudi Arabia contracted MERS from one of his camels. Researchers found that the man and his camel were infected with a genetically identical strain of the MERS virus. About a week before the man became ill, he applied a medication to the nose of the infected camel.

Earlier studies have also found that the majority of dromedary camels in Saudi Arabia have antibodies against the MERS virus.

In the new study, the researchers wrote that the situation with MERS in camels is like that of bird flu (H5N1) in poultry markets in Asia: Although the virus is common in animals, human infection is rare, and can seem random.

Future studies on how the MERS virus spreads from camels to humans should examine whether some people are more susceptible than others to the infection, the researchers said.

The study, which was conducted by researchers at King Faisal University in Saudi Arabia, will be published in the April issue of the journal Emerging Infectious Diseases.

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