This weekend, at least 80 patients had been identified and about 50 suspected cases were investigated.
Diseases have been found in Europe, the USA, Canada, Australia and Israel, among other places.
The United States is preparing to give vaccines to people who have had close contact with those infected.
The United Kingdom has offered vaccines to certain healthcare professionals and the German government has announced that it is investigating the possibilities for vaccination.
However, the immediate availability of vaccines and antiviral drugs is relatively limited, says Richard Pebody, who heads WHO Europe's division for high-risk pathogens, to Reuters.
He believes that infection tracing and quarantine for the sick are sufficient measures to stop the spread of infection.
Not the same risk of infection as covid-19
Smallpox is also not spread as easily as, for example, the coronavirus, and in the current outbreak no one has so far become seriously ill.
It is unclear how the outbreak occurred.
Researchers are trying to find out how the spread of the infection has started and if there is anything in the virus that has changed, but there is still no evidence that the virus has mutated, according to the WHO.