NEW YORK (AP) — Genetic analysis of recent monkeypox cases suggests there are two distinct strains in the United States.

That raises the possibility that the virus has been circulating undetected for some time.

Federal health officials described the genetic analysis Friday.

Many of the U.S. cases were caused by the same strain as recent cases in Europe. But a few samples show a different strain.

Each strain had been seen in U.S. cases last year, before the recent international outbreak was identified.

There have been 20 confirmed or suspected cases of monkeypox in the U.S., according to NBC News. No deaths from monkeypox have been reported in the U.S.

The viral disease can be spread through close contact with someone who is infected.