Updated at 4:30 p.m. ET
The coronavirus can be very serious for anyone at any age but is especially concerning for a man of President Trump's age, 74.
People 65 to 74 years old are five times more likely than younger adults to be hospitalized and 90 times more likely to die, according to the federal Centers for Disease Control and Prevention. Eight out of 10 deaths from COVID-19 in the United States have occurred among older people, the CDC says.
And from what's known about his weight, the president may be officially obese, which is considered another top risk factor.
"Older age and obesity are clear risk factors for developing more severe COVID-19," said Raj Gandhi, an infectious diseases physician at Massachusetts General Hospital and Harvard Medical School. "The president should be closely monitored. "
That said, many people who get infected with the virus do not develop any symptoms — even many of those with risk factors such as their age and weight. Even when people do get sick, lots of them develop cold or flu-like symptoms that are mild. And many people who develop serious complications recover. So there's no way to predict what's going to happen to any specific individual.
"There is quite a broad range for how COVID-19 progresses," Gandhi said. "Some people with COVID-19 never develop symptoms. For those who develop symptoms, they typically do so at about 4-5 days after acquiring the infection but sometimes it takes up to 2 weeks to develop symptoms."
Trump hasn't released as many details about his health as previous presidents have. His doctors have said that despite his age and weight, he is in excellent health. Trump takes medication to lower his cholesterol, but his doctors haven't reported any other health problems that would increase his risk, such as diabetes, heart disease or high blood pressure.
First lady Melania Trump is 50, so she's not in as high a risk group as the president. She underwent a procedure for a benign kidney condition in 2018 but isn't known to have any health problems that would put her at increased risk.
White House chief of staff Mark Meadows told reporters Friday that the president "does have mild symptoms" of COVID-19. The coronavirus can affect people in many different ways. Some people start to feel sick within days. Other people feel fine for longer before developing symptoms. Still others are sick for a while and then seem to be getting better, only to suddenly crash and get seriously ill. That's what happened, for example, to British Prime Minister Boris Johnson. Johnson eventually recovered, but only after receiving intensive care.
While the coronavirus is a respiratory virus, doctors have realized it can affect many different parts of the body, including vital organs aside from the lungs. Some people can develop a variety of complications that can linger for months.
People who are infected are supposed to go into isolation to try to prevent them from spreading the coronavirus. They are advised to stay away from other people and to do things like use a separate bathroom, wear a mask and avoid sharing household items such as cups and towels.
According to CDC guidelines, infected people should isolate for at least 10 days, though that can sometimes be cut short if they get two negative tests at least 24 hours apart. Anyone who develops symptoms is supposed to remain isolated for at least 10 days from when their symptoms first appear — and have gone at least 24 hours without a fever, without taking any fever-reducing medication.
The White House hasn't released details about exactly how the president and first lady plan to handle the logistics of all this in the White House. The White House physician, Sean Conley, released a statement that reads: "The president and first lady are both well at this time and they plan to remain at home within the White House during their convalescence."
As to the president's medical treatment, Conley said in a separate memo on Friday that Trump received a single 8-gram dose of Regeneron's polyclonal antibody cocktail as a "precautionary measure."
"As of this afternoon, the President remains fatigued but in good spirits," Conley said, adding that Trump was being evaluated by a team of experts who would make recommendations about "next best steps."
The first lady has "only a mild cough and headache," he said.
The Trumps and White House staffers were routinely tested to try to prevent the spread of the virus, but their infection shows that this doesn't ensure safety.
"I think the news of the president's and first lady's positive tests show, unfortunately, that a regular testing program doesn't guarantee safety," Jennifer Nuzzo, a senior scholar at the Johns Hopkins Center for Health Security, wrote in an email.