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As physicians across the country diagnose and care for a growing number of people with COVID-19, distinct patterns are emerging, giving clues about how the illness manifests itself in patients.

Very often, people start off with minor physical complaints — slight cough, headache, low-grade fever — that gradually worsen.

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"Patients tend to have symptoms for about a week before either getting better, or getting really sick," said Dr. Joshua Denson, a pulmonary medicine and critical care physician at Tulane Medical Center in New Orleans.

Denson, who estimated he's treated 15 to 20 patients with the coronavirus, described that first phase of the illness as "a slow burn."

Other physicians are seeing similar progression.

"It seems like there's a period of time where the body is trying to sort out whether it can beat this or not," Dr. Ken Lyn-Kew, a pulmonologist in the critical care department at National Jewish Health, a hospital in Denver, told NBC News.

"We're learning about this disease as it's happening, minute-by-minute."

And sometimes, patients start to feel better before their health quickly deteriorates.

"That's what we're seeing with these patients who get a lot worse," Lyn-Kew said. "They're doing OK, and then all of a sudden they're really fatigued, a lot more shorter of breath and having chest pains."

In North Carolina, Dr. Christopher Ohl has also seen rapid, unexpected development of severe symptoms.

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"They say, 'Hey, you know, I think I'm getting over this,' and then within 20 to 24 hours, they've got fevers, severe fatigue, worsening cough and shortness of breath," said Ohl, an infectious disease expert and professor of medicine at the Wake Forest School of Medicine in Winston-Salem, North Carolina. "Then they get hospitalized."

Susan Kane said she noticed her husband, Chris, developed a nagging cough after arriving home from a business trip to Florida late last month. Chris Kane, 55, had no reason to suspect the cough was anything more than a minor cold. He was a non-smoker and had no underlying health conditions.

"It started off as just a little bit of a dry cough," said Susan Kane, who lives in Snohomish County, Washington. "He didn't have any other symptoms but this crazy cough."

But over the next few days, her husband's cough grew worse.

"It ramped up, and then it was coughing and choking and just gasping for air."

A week later, he was diagnosed with coronavirus and hospitalized at Providence Regional Medical Center in Everett, Washington.

"He was really sick," Susan Kane told NBC News. "They put him on oxygen right away."

Kane eventually recovered after being given an experimental treatment.

Though severe coronavirus cases have been reported among younger and middle-aged adults, doctors say older adults, the elderly and those with chronic health conditions seem to be most at risk for the sudden decline.

Denson said nearly all of his most critically ill patients have a combination of three specific underlying medical problems: obesity, high blood pressure and type 2 diabetes.

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The ability to compare notes on patients is useful to physicians who have been thrust into figuring out a brand new virus that's sickened more than 13,000 people in the United States in less than two months.

"We don't have a nice COVID-19 textbook to go back to," Lyn-Kew said. "We're learning about this disease as it's happening, minute-by-minute."

As of Friday afternoon, more than 265,000 cases of the coronavirus have been diagnosed globally, according to Johns Hopkins University.

While severe cases and even deaths occur, they're not the norm.

Some critically ill patients who've needed mechanical ventilation in the intensive care unit have been able to come off the oxygen eventually, and get better. And overall, data on coronavirus cases from China and Europe have shown that more than 80 percent of patients have a mild form of the illness and recover.

Still, doctors said, those most at risk for complications should pay attention to any new symptoms that pop up, even after they start to feel better.

"Be aware of what's going on," Ohl said. "If your symptoms start to get worse after you've been feeling better, then you need to contact your doctor. That's probably something that doctors need to treat in an emergency," he said.

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