“I’m stunned by this. Every day I wake up and there are more countries infected,” said Oyewale Tomori, a virologist who formerly headed the Nigerian Academy of Science and who sits on several World Health Organization advisory boards.

“This is not the kind of spread we’ve seen in West Africa, so there may be something new happening in the West,” he said.

One of the theories British health officials are exploring is whether the disease is being sexually transmitted. Health officials have asked doctors and nurses to be on alert for potential cases, but said the risk to the general population is low.

Outbreaks in Nigeria, which reports about 3,000 monkeypox cases a year, are usually in rural areas, where people have close contact with infected rats and squirrels, according to Tomori. He said the disease is not spread very easily and that many cases are likely missed.