Experts are suggesting that Monkeypox, the virus that has now reached 26 countries, could have been circulating for years without anyone knowing. 

As NBC News reports, scientists and experts are trying to figure out why the virus has suddenly been seen in widespread areas over the past month with hundreds of reported cases. 

“There may have been undetected transmission for a while,” said Dr. Rosamund Lewis, the World Health Organization’s technical lead for monkeypox. “What we don’t know is how long that may have been. We don’t know if it’s weeks, months or possibly a couple of years.”

Virology professor Marc Van Ranst of the University of Leuven in Belgium explains that genetic mutations of the virus that were “limited,” and that he thinks “nobody believes this jumped out of Africa a couple of weeks ago, per NBC. And findings from University of Edinburgh scientists, posted in late May, suggest that their samples came from monkeypox that was discovered in Singapore, Israel, Nigeria, and the United Kingdom between 2017 and 2019. 

“What’s likely happened is an endemic infectious disease from Africa found its way into a social and sexual network and then was greatly aided by major amplification events like raves in Belgium to disseminate around the world,” Dr. Amesh A. Adalja, a senior scholar at the Johns Hopkins Center for Health Security, said.

Adalja added that “because it’s being transmitted through close contact in sexual encounters,” diagnosis may be delayed because “many of the lesions are getting mistaken for other sexually transmitted infections.”

As previously reported, Dr. Raj Panjabi, the Senior Director for Global Health Security and Biodefense at the National Security Council, told NPR that the infection has not been severe in the 10 or so cases seen in the U.S., as of last week. The infection—which has seen 25 cases reported in the Montreal area–often spreads via skin-to-skin contact if someone has rashes or lesions, and only one case so far in the U.S. has been confirmed as of last week over in Massachusetts.

“Historically in countries with weaker health care systems less than 1 percent of patients have died from this milder strain,” Panjabi said. “We have access to vaccines and even treatments here in the U.S., and so the risk we believe is substantially lower.”