Portuguese authorities said on Wednesday they had identified five cases of the rare monkeypox virus, and that Spanish health services were testing 23 possible cases after Britain put Europe on alert for the virus.

Of the 20 suspected cases in the country, the five Portuguese patients have been stable. They are men and they all live in the Lisbon region and the Tagus Valley, according to Portuguese health authorities.

European health officials have been monitoring any outbreaks of the disease since the UK reported its first case of monkeypox on May 7 and have found six more cases in the country since then.

Spain issued an alert early this morning saying it had eight suspected cases under testing. Madrid region health officials said in a statement that the number had risen to 23 in the late afternoon. All cases remain unconfirmed.

Monkeypox is a rare viral infection similar to human smallpox, although milder, first recorded in the Democratic Republic of the Congo in the 1970s. The number of cases in West Africa has increased over the past decade.

Symptoms include fever, headache, and a rash that begins on the face and spreads to the rest of the body.

Spanish health officials said it’s not particularly contagious among people, and most infected people recover within a few weeks, although serious cases have been reported.