A growing outbreak of monkeypox cases has spread across several countries, including the US, suggesting that the animal-transmitted disease that occurs in forested areas of Central and West Africa has been quietly spreading undetected.

Cases of the smallpox-related disease have previously been seen only among people with links to central and West Africa and scientists say they are baffled by the disease’s recent spread in Europe and North America.

About 80 cases have now been confirmed across 13 countries with the majority of infections in Spain linked to a sauna in Madrid.

So far, the US has reported one case in a Massachusetts man who had recently travelled to Canada, which, as of Thursday, reported 17 suspected cases in Montreal. The United Kingdom has identified nine cases, one of which is connected to recent travel to Nigeria, where monkeypox is endemic. But the other cases appear to have been infected within the UK and are all not linked to the travel-related case by contact or timing. Portugal is investigating more than 20 cases, Spain is reportedly investigating 23 cases, and Italy and Sweden have each reported at least one case.

There are about 80 confirmed cases worldwide and 50 more suspected ones, the World Health Organisation (WHO) said.

In Spain, 24 new cases were reported on Friday, mainly in the Madrid region where the regional government closed a sauna linked to the majority of infections.

Monkeypox typically causes fever, chills, rash and lesions on the face or genitals. To date, no one has died in the outbreak.

The disease, which was first found in monkeys, can be transmitted from person to person through close physical contact and is caused by the monkeypox virus.

Contact with live and dead animals – for example through hunting and consumption of wild game or bush meat are known risk factors.