The British health authorities say they have identified four “rare and unusual” cases of monkeypox among men who appear to have been infected in London and have not traveled to African countries where the smallpox-like disease is endemic.

In a statement Monday, the UK Health Agency said it had launched an investigation into how the men became infected and whether they had any contact with each other. Three men are in London and one in the north-east of England. All men identify themselves as gay, bisexual or men who have sex with men.

Last week, the UK reported three previous cases of monkeypox, including two people living in the same family and another who had previously traveled to Nigeria, where the disease is endemic to animals.

“Evidence suggests that the monkeypox virus may be transmitted in a community that spreads through close contact,” said Dr Susan Hopkins, chief medical adviser to the UK Health Agency. “We especially urge gay and bisexual men to be aware of any unusual rashes or lesions and to contact the sexual health service immediately.”

Smallpox of monkeys is a viral disease usually spread by touching or biting the infected wild animals like rats or squirrels in West and Central Africa. The disease is not usually easily transmitted between people, but British health officials say it is possible if there has been very close contact with an infected person.

It is not known that monkeypox is a sexually transmitted disease.

The British authorities said that due to the recent outbreak of cases and uncertainty in how monkeypox is transmitted, they are working with hospitals and international partners to find out if there has been a similar increase in cases elsewhere. Public health Experts said that doctors treating patients who have a rash “without a clear alternative diagnosis” should seek professional advice.

They said the risk is for population as a whole was low, and that infected patients received specialized care in London and Newcastle. Officials are tracking down those who have had contact with sick monkeys, including plane passengers.

Early symptoms smallpox monkeys include fever, muscle aches, chills and fatigue. In more severe cases, a rash may develop, often on the face and genitals, resembling a rash seen with chickenpox and smallpox. Most people recover from the disease within a few weeks.

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