NEW YORK (NYTIMES) - When the first monkeypox cases were identified in early May, European health officials were stumped. The virus was not known to spread easily among people, let alone infect dozens - and soon hundreds - of young men.

The origins of the outbreak are now becoming clearer. Genetic analysis suggests that although the monkeypox virus is rapidly spreading in the open, it has been silently circulating in people for years.