The World Health Organization expressed concern Wednesday about the recent sharp rise in MERS cases in Saudi Arabia and the United Arab Emirates.

Between them, the two countries have announced nearly 150 new infections with the MERS coronavirus this month. In fact, the cases announced so far in April represent 43 per cent of all MERS cases diagnosed since the first known infections in April of 2012.

It took two years since those first cases for the global tally to hit the 200 mark. That happened in late March. Now the combined total announced by the WHO and affected countries is over 360 cases.

The WHO statement said it is particularly worried about two outbreaks in health-care facilities, calling them significant. One is occurring in Jeddah, Saudi Arabia, while the other is in Abu Dhabi in the United Arab Emirates.

While the UAE has announced a couple of dozen cases in recent days, the lion's share of the spike has occurred in Saudi Arabia, which has announced 80 new cases in the past week.

Earlier this week Saudi Arabia's executive council fired the country's health minister. While no official reason was given, it is believed dissatisfaction with his handling of the MERS outbreak was behind the move.

The WHO noted about three quarters of the recently diagnosed cases involve person-to-person spread of the virus, with most of the affected people contracting the virus in hospitals.

"Approximately 75 per cent of the recently reported cases are secondary cases, meaning that they are considered to have acquired the infection from another case through human-to-human transmission," said Dr. Ala Alwan, regional director of the WHO's Eastern Mediterranean Regional Office.

"The majority of these secondary cases have been infected within the health-care setting and are mainly health-care workers, although several patients are also considered to have been infected with MERS-CoV" -- CoV is short for coronavirus -- "while in hospital for other reasons."

The two countries have also exported cases. A man from Greece who became ill with a gastro-intestinal ailment while in Jeddah sought care in a hospital there. Later he flew home with symptoms of MERS and has been confirmed as a case.

A man from Malaysia who was in Saudi Arabia on a religious pilgrimage contracted the virus and died after returning to his home country. And a nurse who tested positive for the virus in UAE flew home to the Philippines.

Among the 24 cases that Saudi Arabia reported Wednesday was a Turkish man who was in the country as a pilgrim. The Saudi ministry of health says the man is in hospital in Mecca in stable condition.

Until recently, Saudi officials have reluctantly shared information, preferring, it appeared, to deal with the problem on their own. The WHO and outside experts have plead with authorities there to conduct what's known as a case control study to try to figure out how an animal virus -- likely coming from camels -- is infecting people. But to date there has been no such study done or if one has been done its results have not been shared.

But in recent days Saudi officials have spoken of seeking help from the WHO and international experts. And WHO said Wednesday it has offered to mobilize international expertise through its Global Outbreak Alert and Response Network to help both Saudi Arabia and the UAE.

One expert who has been following MERS developments closely is Dr. Allison McGeer, an infection control expert and a key figure in Toronto's SARS outbreak response.

McGeer, who is head of infection control at Toronto's Mount Sinai Hospital, said it is hard from a distance to gauge what going on with MERS, but she expressed concern about the apparent size of the Jeddah outbreak.

McGeer travelled to Saudi Arabia in the spring of 2013 to investigate and help control an earlier hospital outbreak at Al Ahsa. She said containing the virus there was easy -- which leads her to wonder why the Jeddah outbreak appears to be so large and difficult to stop.

She said one possibility is that infection control measures are failing. But McGeer said Saudi Arabia has good infection control expertise.

Another option, she noted, is that something may have changed in the virus, making it more transmissible than it was last year. But she said there isn't evidence to suggest the virus has changed enough to be able to spread efficiently from person-to-person in the community, like cold and flu viruses do.

Still, SARS did most of its damage in hospitals, where it spread well among patients and health-care workers. And people familiar with SARS who are watching MERS see many parallels between the two.

"It is worrying me that the hospital control of transmission appears to be a significant problem," said McGeer. "It suggests that the virus is changing and if the virus is changing then that of course doesn't rule out that we could have a new human coronavirus...if we just give it enough time."