About 80 cases of monkeypox have been confirmed in 11 countries, the World Health Organization says, warning that more cases are likely to be reported.
The WHO says that another 50 suspected cases are being investigated, without naming any countries.
Monkeypox is most common in remote parts of Central and West Africa.
It is a rare viral infection which is usually mild and from which most people recover in a few weeks, according to the UK's National Health Service.
The virus does not spread easily between people and the risk to the wider public is said to be very low.
There is no specific vaccine for monkeypox, but a smallpox jab offers 85% protection since the two viruses are quite similar.
The first case of the disease in the UK was reported on 7 May. The patient had recently travelled to Nigeria, where they are believed to have caught the virus before travelling to England, the UK Health Security Agency said.
Authorities in the UK said they had bought stocks of the vaccine and started offering it to those with "higher levels of exposure" to monkeypox.
Australia's first case was detected in a man who fell ill after travelling to the UK, the Victorian Department of Health said.
In North America, health authorities in the US state of Massachusetts confirmed that a man has been infected after recently travelling to Canada. He was in "good condition" and "poses no risk to the public", officials said.