The WHO warned that monkeypox may have been spreading under the radar for some time, even as cases rose to 550, in a statement on Wednesday, 1 June.
"Investigations are ongoing, but the sudden appearance of monkeypox in many countries at the same time suggests there may have been undetected transmission for some time."Tedros Adhanom Ghebreyesus, Director-General, World Health Organization
The WHO said there have been more than 550 confirmed cases of the disease, in 30 countries outside of West Africa and centrally endemic areas, since Britain first reported a confirmed case of monkeypox on 7 May.
Rosamund Lewis, the WHO's monkeypox lead, stated that the sudden appearance of so many new cases in Europe and other countires "is clearly a cause for concern, and it does suggest undetected transmission for a while".
"We don't know if it is weeks, months, or possibly a couple for years," she said, adding that "we don't really know if it is too late to contain".
Monkeypox is related to smallpox, which killed millions of people worldwide before being eradicated in 1980.
Must Fight the Stigma, Says WHO
However, monkeypox, which is transmitted through close contact, is less severe, and symptoms usually include a high fever and a chickenpox-like rash that clears up after a few weeks.
So far, most cases have been reported in men who have had sexual relations with another man, but experts stress that there is no evidence that monkeypox is a sexually transmitted disease.
"Anyone who has close physical contact with another infected person can get monkeypox," Tedros said.
"Not only is it wrong to fight stigma, but it can also discourage patients from seeking treatment and make it difficult to stop infection."Tedros Adhanom Ghebreyesus
Previously, WHO had urged affected countries to "increase surveillance". Lewis had emphasised the importance of working together in order to prevent any future epidemics, stressing on contact tracing and isolation of infected patients.
While smallpox vaccines have proved to be 85 percent effective against monkeypox, they are in short supply and thus, cannot be fully relied on.
Targeted use of vaccines for healthcare workers or people at the most risk of the infection rather than mass vaccination is WHO's plan.
Endemic countries that witness thousands of people falling prey to this infection every year, have also reported a rise in the number with about 70 deaths in five african countries so far.
Amongst the cases found outside of the endemic areas, no death has been reported yet.
The fatality rate for monkeypox is believed to be usually low. However, Maria Van Kerkhove, Lead, Emerging Diseases at the World Health Organization (WHO), warned that the virus could turn deadly if it infects a more vulnerable population.