By Holly Yan and Betsy Klein, CNN

(CNN) -- If you're in a coronavirus hotspot, watch out -- Dr. Deborah Birx might be visiting and reinforcing some sharp lessons.

The White House coronavirus task force coordinator has been traveling across the country in recent weeks, meeting with state and local officials as the fall Covid-19 surge grips all regions of the country.

North Dakota had the highest rate of new cases in the US in the most recent task force report for the state, dated October 18. And during her visit to North Dakota on Monday, Birx slammed the inadequate mask usage in the state.

"Over the last 24 hours, as we were here and we were in your grocery stores and in your restaurants, and frankly even in your hotels, this is the least use of masks that we have we seen in retail establishments of any place we have been," Birx told local reporters, according to The Bismarck Tribune.

"And we find that deeply unfortunate because you don't know who's infected, and you don't know if you're infected yourself."

North Dakota is one of 15 states that does not have a mask mandate.

Health experts from across party lines have said wearing face masks is one of the easiest ways to help keep the economy open.

And If 95% of Americans wore masks in public, more than 100,000 American lives could be saved through February, according projections Friday from the University of Washington's Institute for Health Metrics and Evaluation.

"When you look at the states and when they implemented a mask mandate and you look at their cases per million and you look at their fatalities per million, they are lower than states that chose to recommend masks but not to mandate," Birx said, according to the Bismarck Tribune.

Masks are especially important now, as cooler weather leads to more people socializing indoors -- where coronavirus can spread more easily.

"People let down their guard when they were with friends and family, and they took off their masks, and they share dinner, or they share drinks inside, and those become spreading events," Birx said earlier this month during a visit to Massachusetts.

Most states are headed in the wrong direction with new Covid-19 infections.

As of Tuesday, 37 states had at least 10% more new cases this past week compared to the previous week, according to data from Johns Hopkins University.

Only one state, Washington, had a decrease of at least 10%. The remaining 12 states are relatively steady.

The nationwide surge isn't just due to more testing -- in fact, the increase in Covid-19 cases far outpaces the increase in testing.

The average number of daily new cases this past week is up 20% compared to the previous week, according to Johns Hopkins.

But testing has increased only 3.64% over the same time frame, according to the Covid Tracking Project.