The agency also suggested investigating clusters of severe respiratory infections, and clusters of such illnesses in health-care workers, regardless of where they happen in the world.
Up until now the WHO has said that testing for the new coronavirus should be restricted to patients with severe respiratory infections who had recently travelled to or who were residents of a country that had recorded cases. To date the only confirmed infections have been in Qatari and Saudi nationals.
That change in advice, the basis for which the WHO did not explain, raised eyebrows among some infectious diseases experts, who were quick to try to read between the lines.
"That suggests that they have the idea that it's more widespread. Where does that idea come from? What's the evidence?" wondered Dr. Ron Fouchier.
The Dutch virologist leads the laboratory which in June found that a new coronavirus — from the same general family as the virus that caused SARS — was behind the infection of the first identified case, a man from Jeddah, Saudi Arabia.
If the WHO has any evidence that the virus has spread further afield, it hasn't revealed it.
But its concern may stem from the fact that over three million Muslim pilgrims have recently returned to their home countries after attending this year's Hajj, which ended in late October.
The WHO's new advice was contained in a statement the agency released Friday in which it announced the global count of confirmed infections with the new virus has risen to six. Two of the confirmed cases have died. All six cases were male.
It also revealed that Saudi Arabia has reported a cluster of cases, with two men confirmed and two others under investigation. The four men shared a household in an undisclosed part of the country.
All four were sick around the same time, suffering similar symptoms, the WHO said. Of the two men under investigation, one died. Test results are still pending on samples taken from the man during his illness.
The other man survived and tested negative for the virus, the WHO said. But it did not disclose the type of test used or when the testing was done.
Until more information is known, it is not clear that the test result can be considered reliable, said Dr. Michael Osterholm, director of the Center for Infectious Diseases Research and Policy at the University of Minnesota.
The reliability of the test could vary, depending on when the person was tested, what kind of test was used and the kind and calibre of the specimen being tested, he said.
"If the person had an illness similar to the other illnesses, then ... I believe that you'd have to consider that this test may have been a false negative," said Osterholm, adding testing the survivor's blood for antibodies would shed some light on the situation.
Both the changing WHO advice and the lack of clarity on the testing underscore a problem with this situation, both Osterholm and Fouchier suggested.
Very little about the cases is being publicly shared. And the international teams of scientists who travelled to Saudi Arabia to look for possible sources of the virus have released no information about their investigations.
Fouchier was front and centre in the laboratory effort during the 2003 SARS outbreak. It was his lab, at the Erasmus Medical Centre in Rotterdam, that proved what's called Koch's postulates — the test that confirmed that the newly identified coronavirus was actually causing the disease SARS.
During the early days of the SARS outbreak, the WHO rapidly put together a virtual network of laboratories, tapping into expertise around the world to combat the alarming new disease.
But this time? In the summer, Fouchier's lab identified and sequenced the new coronavirus and developed a test for it. But since then, it's been "radio silence," Fouchier said in an interview Friday.
"Everything I've heard since then has just come from the lay press, which is completely in contrast to how we acted back in the SARS era," he said.
"That was completely different during the SARS outbreak. We were all talking together, exchanging results and giving each other ideas about what to test, how to test, where to test. And none of that is happening now. We just have to rely that they're doing the right thing."
Officials in the know should be sharing more information, Osterholm agreed.
"At this point in any outbreak investigation, there clearly is more information that is known by health officials than likely has been shared," he said.
"But if there were ever a time for complete transparency, now is the time. We've learned that in the past and I'd hate to see us have to relearn the lesson again."