The next five to 10 days will be critical for President Donald Trump after his positive test for COVID-19. Doctors warn the illness can suddenly worsen after several days of relatively mild symptoms.
That's particularly true for patients with two of the biggest risk factors for serious and even life-threatening complications of the disease: obesity and older age. At 74 and with a body mass index of 30.5, qualifying him as obese, Trump fits both criteria.
"I'm worried," Dr. Vin Gupta, a pulmonologist and an affiliate assistant professor at UW Medicine in Seattle, said Friday on NBC's "TODAY."
"I've cared for several patients with that double whammy," Gupta said. "If they become sick, if they are symptomatic, they can escalate quickly to needing ICU care."
What's more, research has shown men tend to fare worse from COVID-19 than women.
White House chief of staff Mark Meadows told reporters Friday that the president was experiencing "mild symptoms" but declined to elaborate. First lady Melania Trump tweeted that she, too, has mild symptoms but is "overall feeling good." A letter from the White House physician confirming the positive test results for both Trumps said they were "well at this time."
Only time will tell how the virus will affect them. "The hallmark of this infection is unpredictability," Dr. Michael Saag, an associate dean for global health at the University of Alabama, said. "It is impossible to know once someone gets infected what's going to happen to them clinically."
Saag not only treats COVID-19 patients; he had a serious case of the illness in March.
"The most common symptom is profound fatigue. Having experienced this infection myself, I can underscore the word profound," Saag said, adding that fatigue is generally the longest-lasting symptom.
"I would predict that if he's going to have symptoms, he will be suffering with some degree of fatigue from now through Election Day."
It is unknown when the president was infected. That knowledge would be key in directing how his physicians will monitor and potentially treat him.
"I try to get as good a handle as I can on when my patient contracted the virus," said Dr. Pieter Cohen, a physician with the Cambridge Health Alliance Respiratory Clinic near Boston. "If the president was exposed to the coronavirus less than five days ago, then this is very early in the infection."
Cohen and other doctors who have been caring for COVID-19 patients for months say the infection has two key parts: an initial phase with minimal to no symptoms, followed, for some, by a more severe phase.
"Then there is the later, more concerning, days of illness, usually days four through eight but can be later as well," Cohen said. "That is when serious complications, often beginning with difficulty breathing, tend to begin."
Dr. Josh Denson, a pulmonary medicine and critical care physician at the Tulane Medical Center in New Orleans, said it's rare for a person to become very sick within the first few days of infection.
"This could drag out," Denson said. "If he recovers, it won't be for a week or so. But I've had some patients get critically ill more than two to three weeks after the initial infection."
The Centers for Disease Control and Prevention's guidelines indicate the president should be in isolation for at least 10 days, and should remain in a "sick room or area and away from other people."
It is likely that during that time, Trump's oxygen levels will be carefully monitored using a device called a pulse oximeter, Gupta said. Monitoring oxygen levels is critical as they can plummet in people with COVID-19, even if they don't feel sick.
"His physicians will be checking his oxygen level. Is he short of breath? Does he have a fever? Does he have chills or a cough? Extreme vigilance here is warranted," he added.
Saag also said that shortness of breath, especially when walking, could be a key predictor for a complication like pneumonia. Patients on the verge of entering that second phase of illness may develop a dry cough.
"I can tell because it's hard to get an exam done with these patients. I'll say, 'take a breath' and they'll get about halfway through and just start coughing," Saag said.
Other symptoms may include difficulty concentrating and chest pain.
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Doctors may treat any symptoms the president may develop, but there is no known treatment proven to prevent COVID-19 patients from moving from the initial stage of illness to the second, potentially more dangerous stage, Cohen said.
It is possible that Trump's doctors could offer him remdesivir, convalescent plasma or a similar type of treatment called monoclonal antibodies. All of those require intravenous infusions, however, and remdesivir is given over a five-day period.
The Food and Drug Administration has issued emergency use authorizations for both remdesivir and plasma, but studies of those treatments are still ongoing. With potential benefit comes potential risk.
"Like a lot of things with this epidemic," Saag said, "this is a giant experiment."
This story originally appeared on NBCNews.com.