(CNN)An extremely rare disease called monkeypox, a much less severe cousin of smallpox, is spreading around the world. More than 250 cases have been reported in at least 16 countries, according to the World Health Organization. Experts say it is spread by close and prolonged contact with an infected individual.

“The countries that are reporting monkeypox now are countries that do not normally have outbreaks of monkeypox,” Rosamund Lewis, head of the smallpox secretariat, WHO Emergencies programme, said Tuesday during a news briefing at the United Nations in Geneva.