For the first time this week, new infections reported around the world have surpassed those in mainland China, where the flu-like disease emerged two months ago from an illegal wildlife market but is on the decline after an aggressive containment campaign.
Though meeting the dictionary definition of a pandemic - widespread contagion across a large region - the WHO has so far held back from using that term.
"This virus has pandemic potential," WHO chief Tedros Adhanom Ghebreyesus told reporters in Geneva.
"This is not a time for fear.
This is a time for taking action to prevent infection and save lives now."