The virus, which causes lesions, fever, muscle ache and chills, has typically spread to people from infected animals like rodents, although human-to-human transmission has been known to occur.
Spain and Portugal have detected around 30 suspected cases of monkeypox, after British authorities found several cases of the viral infection that is rare in Europe.
In Portugal, 20 suspected cases of monkeypox — endemic in parts of Central and Western Africa — have been detected in the Lisbon region, the health ministry said in a statement on Wednesday.
"The cases were all among males, the majority of them young, who had ulcerated lesions," it added.
All of them were reported to be in stable condition. Authorities did not say if the men had a history of travel to Africa or any links with recent cases in Britain or elsewhere.
In Spain, authorities have detected eight suspect cases of the smallpox-like disease that "still must be confirmed" by analysis, the health ministry said.
Cases in the UK
The UK Health Security Agency (UKHSA), a public health protection body, said on Monday it had detected four new cases after registering three cases earlier in May.
The next day the World Health Organisation said it was coordinating with the UK to investigate the outbreak.
Symptoms of monkeypox in humans include lesions, fever, muscle ache and chills. Most people recover from the illness within several weeks.
Transmission is usually via close contact with infected animals such as rodents and monkeys, and is limited between people.
Among people, the disease is spread when there is very close contact with lesions, body fluids, respiratory droplets or contaminated materials, like bedsheets. It has only been fatal in rare cases.
The disease belongs to a family of viruses that includes chickenpox and smallpox.
A vaccine developed against smallpox has been approved for monkeypox, and several anti-virals also appear to be effective.
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