SEATTLE — Public gatherings of over 250 people are banned for the rest of the month in Snohomish, King and Pierce counties, Gov. Jay Inslee announced Wednesday in an extraordinary new effort to stifle the spread of the new coronavirus.

Leaders of King and Snohomish counties plan to order even stricter prohibitions on smaller events of 250 people or less.

Inslee’s proclamation affects nearly 4 million residents of the Seattle metro area. It does not force the closure of schools, businesses or markets. However, schools in the region have been warned to prepare for potential mandatory closures.

“This is not just your ordinary flu,” Inslee said Wednesday morning at a news conference in Seattle. “This demands a response consistent with the nature of the threat.”

Inslee’s order prohibits “social, spiritual and recreational activities including, but not limited to, community, civic, public, leisure, faith-based, or sporting events; parades; concerts; festivals; conventions; fundraisers; and similar activities.”

Under state law, the governor may issue an order banning activities he “reasonably believes should be prohibited to help preserve and maintain the life, health, property or the public peace.”

Washington has documented at least 25 deaths related to the new coronavirus and at least 267 infections.

Projections suggest that inaction could allow the number of infections to top 25,000 by early April, said Jeff Duchin, health officer for Public Health for Seattle and King County.

In the two Washington counties most affected by the virus, King and Snohomish, all gatherings are restricted “unless they meet public health guidelines to ensure social distancing, adequate sanitation, regular health checks of employees, and other measures designed to prevent the virus from being transmitted,” King County Executive Dow Constantine said.

Snohomish County Executive Dave Somers said that plan would be mirrored within his jurisdiction. That plan that will be handled by the Snohomish Health District, according to the county executive’s office.

Inslee’s mandate is in place through March, but it’s very likely that it will be extended, the governor said. The order may also expand beyond the borders of the three counties.

“It is clear that our state needs a more vigorous and more comprehensive and more aggressive position if we are going to slow the spread of this epidemic,” Inslee said.

The 250-person limit was chosen after consulting with scientists and local leaders, Inslee said.

“There is no magic number written in the stone tablets,” he said.

The new rules do not apply to airports, the governor said. For large companies like Microsoft, Amazon and Boeing, the governor urged employers to have workers telecommute wherever possible. Across the country, some have already told employees to work remotely, such as Google.

Seattle Mariners baseball, Seattle Sounders soccer and Everett Silvertips hockey games will be affected by the governor’s order. So will graduations, weddings, festivals, concerts and worship services.

The state is ready to enforce the order if its violated, Inslee said. He doesn’t anticipate any issues with compliance.

The governor did not order school closures, but said he is recommending that all school districts prepare for the possibility of mandatory closures that could last weeks or months, which could come in days.

Nineteen deaths are linked to a Kirkland nursing home, and authorities said the virus spread to at least 10 long-term care facilities in the Seattle area. Snohomish County health officials announced Tuesday that three residents of the Josephine Caring Community in Stanwood had tested positive.

On Wednesday, King County had 190 confirmed cases, Snohomish reported 60 and Pierce had 14.

The World Health Organization declared the worldwide outbreak a pandemic on Wednesday.

Duchen likened the effect of the virus to a major earthquake.

“We expect a large-scale outbreak in weeks,” he said. “This is going to be a very difficult time.”

The governor pointed to the situations in Italy and China as possibilities for what the U.S. could have in store.

In Italy, the death toll from the virus is nearing 1,000 people, according to multiple reports.

“Today’s actions will help relieve the strain on our hospital system,” Constantine said.

Seattle Mayor Jenny Durkan said, “We will get through it, it will be hard.”