The World Health Organization WHO has said that hundreds of monkeypox cases have surfaced beyond the African countries, warning that the virus has likely been spreading under the radar.

WHO Director-General Tedros Adhanom Ghebreyesus told reporters on Wednesday that 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.

The United Nations health agency’s top monkeypox expert Rosamund Lewis said that the appearance of so many cases across much of Europe and other countries is clearly a cause for concern.