The incubation period for the new MERS coronavirus may be longer than has been believed up until now, a new study suggests.

French doctors report that a man who caught the virus from an infected patient he shared a hospital room may have gone 12 days before developing symptoms.

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People who have symptoms of the disease and have travelled to the Middle East or have been in contact with a known MERS patient within the past 12 days should be isolated, French doctors say. (Denis Charlet/AFP/Getty)

They suggest people who have symptoms of the disease and have travelled to the Middle East or have been in contact with a known MERS patient within the past 12 days should be isolated and investigated as a possible case.

The work is published in the journal The Lancet.

Knowing the incubation period of a disease is critical; it helps doctors spot cases and public health officials control spread of disease by isolating those who might be becoming ill.

But the incubation period for MERS still isn't known because so little information on cases has been publicly shared to this point.

To date there have been 49 confirmed cases and 27 of them have died. Infections appear to originate from countries on the Arabian Peninsula, with Saudi Arabia reporting the lion's share of illnesses. Cases have been exported to France, Britain, Germany and Tunisia.

The first case in France's cluster of two cases was a man who travelled to the United Arab Emirates in April on a package tour. He became ill when he returned home. The man died Tuesday.

Before the nature of his illness was determined, he shared a hospital room with another man. The French doctors reported that neither man wore protective masks when they shared the room.

The second man was treated for his original medical problem and was sent home, only to return eight days later with respiratory symptoms. The men shared a room for three days, so it's impossible to say exactly when the second man became infected.

But the dates suggest his incubation period could have been as long as 12 days, which is lengthier than has been estimated up until now.

For instance, the ministry of health in Saudi Arabia, in a frequently-asked-questions section on their coronavirus website, says the incubation period of other known coronaviruses is roughly a week "and we think it's mostly the same."

The World Health Organization has been publicly pressing Saudi officials to be more forthcoming with information about their now 37 MERS cases, saying only by pooling data can the world assess how the virus infects, what the period of infectivity is, what the incubation period is and how big a risk it poses.