PA Governor Tom Wolf

Gov. Tom Wolf of Pennsylvania speaks at a new conference at Pennsylvania Emergency Management Headquarters about COVID-19, Thursday, March 12, 2020 in Harrisburg, Pa.

Pennsylvania Gov. Tom Wolf is ordering residents of the state's hardest-hit areas to stay home to potentially save a life and help combat the spread of the new coronavirus that has already sickened hundreds and caused six deaths statewide.

Wolf announced the stay-at-home order -- which is in addition to an existing order in Philadelphia -- for Bucks, Chester, Delaware and Montgomery counties, surrounding Philadelphia; Allegheny County, which includes Pittsburgh; and Monroe County in the Pocono Mountains.

The goal is to prevent a big strain on the state's health care system.

"Don't leave your home unless someone's life depends on you leaving, because, ultimately, someone's life does depend on you staying," Wolf said Monday afternoon.

Together, the stay-at-home counties account for about 75% of the state’s confirmed cases of COVID-19.

Wolf said he hopes the stay-at-home order in those counties will help prevent COVID-19 from expanding in other parts of the state.

At least 644 people have tested positive for COVID-19, with six deaths, including one in Montgomery County and two in Northampton County.

"This virus, you know, is sneaky," Wolf said. "You get it before you know you have it. Now, 80% percent who get it, they're going to be OK. And if you're part of that 80% you can be forgiven for saying, 'What's the big deal?' The big deal is that 20% are in danger. And that 20% includes our loved ones and our neighbors. And our problem right now is that we cannot treat that 20% if the virus takes off as it did in Italy. We don't have the capacity in our health care system so we need to buy time."

The stay-at-home order is the latest in a series of progressively tougher measures imposed by Wolf in the face of a pandemic that state officials say threatens to swamp hospitals and spike the death toll. Wolf had already closed schools and ordered all “non-life-sustaining” businesses to shutter their physical locations, an edict that state police and other government agencies began enforcing Monday morning.

Businesses that remain open to the public include grocery stores, pharmacies, hotels and motels, beer distributors, laundromats and gas stations. Restaurants are only open for take-out orders. The open list also includes farms, mines, food production and some manufacturing.

Car dealers, clothing stores and other retailers, salons and entertainment venues are among those on the shuttered list.

Wolf is also extending the statewide closure of schools for two more weeks, starting next week.