Monkeypox is a usually mild viral infection. Symptoms include fever, headaches and skin rashes.
Several cases have been detected in Britain - where authorities are offering a smallpox vaccine to healthcare workers and others who may have been exposed - and others in other parts of Europe, including in Spain's neighbour Portugal.
Monkeypox occurs in central and west Africa, often in proximity to tropical rainforests, and is considered endemic in the Democratic Republic of Congo, where it was first discovered in humans in 1970.
The World Health Organisation, already busy with the coronavirus pandemic which has struck the world over the past three years, has called for vigorous contact tracing of the spate of cases.
So far, four countries in Africa have reported cases of monkeypox in 2022, namely Cameroon, Central African Republic, Democratic Republic of Congo and Nigeria, the WHO said.
The virus usually causes symptoms similar to smallpox but milder.
Cases can also be severe.
Smallpox has been considered eradicated worldwide since 1980 after a major vaccination campaign.
Experts suspect that the pathogen that causes monkeypox circulates in rodents - monkeys are so-called false hosts.
According to the WHO, previous cases of monkeypox were usually due to travel to areas in west and central Africa.
The WHO said monkeypox has appeared more frequently in Nigeria in recent years.
Since 2017, a total of 558 suspected cases have been reported there.
Of those, 241 have been confirmed and eight people have died.