A step-by-step flow chart breaks down how coronavirus cases typically progress — and shows what may still be ahead for Trump

• President Donald Trump left the Walter Reed Medical Center on Monday evening after doctors said his condition had improved.
• Trump tested positive for the coronavirus on Thursday. By Friday, he had a fever and his blood-oxygen level had dropped at least once, so he was hospitalized.
• It's common for coronavirus cases to wax and wane. But many patients take a turn for the worse around day 10 of symptoms.
• Trump's illness could follow that standard progression in the coming week.

As a 74-year-old, overweight male, President Donald Trump was more vulnerable to a severe COVID-19 infection than someone younger and more spry would be. So it was little surprise when, after testing positive for the virus on Thursday, his condition went downhill.

White House chief of staff Mark Meadows told the Associated Press that Trump had a "very concerning" period on Friday. The president's blood-oxygen level dipped below normal, and he developed a high fever. Trump's doctors confirmed on Sunday that the president has gotten supplemental oxygen twice.

That's a sign of a more severe case of COVID-19. On Wednesday afternoon, White House physician Dr. Sean Conley said Trump has been "symptom-free for over 24 hours."

If Trump's first symptoms appeared the day he tested positive, the typical progression of the illness would suggest he is approaching a crucial moment when coronavirus cases can take a turn for the worse: day 10 of symptoms. That's around the time when severe cases are admitted to the hospital.

We know Trump had reached at least day seven by Wednesday, but the White House hasn't clarified exactly when his symptoms began. With that assumption, his day 10 could hit on Saturday.

Trump, of course, isn't an average patient: He has received a level of attention and access to treatments thus far that almost nobody else can get.

The president was admitted to Walter Reed Medical Center on Friday evening, fewer than 24 hours after testing positive. Treating symptoms early often improves the chances of a patient's survival. Trump also received an experimental antibody cocktail that isn't publicly available yet. And he has been treated with dexamethasone, a steroid usually reserved for the sickest coronavirus patients.

All of these factors likely make Trump's prognosis better than other men in their 70s. But his doctors still said the president isn't out of the woods, and that his illness could follow a standard chain of events in the coming week. Here's what that looks like, based on data so far.

Most patients don't reach the ICU stage

As many as 40% of coronavirus cases are asymptomatic, according to the Centers for Disease Control and Prevention.

Among patients who develop symptoms, a fever and cough are usually the first to arrive. They're often followed by a sore throat, headache, muscle aches and pains, nausea, or diarrhea (though, in severe cases, gastrointestinal issues can appear earlier in the course of an infection).

Severe cases usually develop difficulty breathing — one of the virus' hallmark symptoms — around five days after symptoms start.

But it could be another two to six days before patients are admitted to the hospital with conditions like pneumonia or sepsis. Around this time, patients may also experience clotting in their brain or lungs.

Scientists have a few theories about why some coronavirus patients take a rapid turn for the worse. One is that immune systems overreact by producing a "cytokine storm" — a release of chemical signals that instruct the body to attack its own cells.

Dr. Panagis Galiatsatos, a pulmonary physician at Johns Hopkins Bayview Medical Center, compared that process to an earthquake — generally, it's the falling buildings that kill someone, not the quake itself.

"Your infection is a rattling of your immune system," he said. "If your immune system is just not well structured, it's just going to collapse."

In severe cases, an aggressive immune response can lead to acute respiratory distress syndrome, which may necessitate a ventilator. This often happens around day 12 of symptoms. That's also around the average time when critical COVID-19 patients are admitted to the ICU.

A 'horrible purgatory' between death and recovery

But viewing coronavirus infections based on averages can hide the fact that the disease often doesn't progress in a linear fashion.

"Courses can step by step worsen progressively. They can wax and wane, doing well one day, worse the next," Megan Coffee, an infectious-disease clinician in New York City, told Business Insider. "An 80-year-old man with medical issues can do quite well. Sometimes a 40-year-old woman with no medical issues doesn't."

Coffee estimated that one in four hospitalized COVID-19 patients wind up on the ICU track. The rest, she said, remain in a standard hospital ward, then get discharged after a week or two. (Trump was discharged after 72 hours.)

If a patient's immune system continues to overreact, they may experience damage to other organs, like the heart or kidneys, by the second week of symptoms. Among patients whose health deteriorates quickly and critically, death can come around day 18 of symptoms.

But therapies like the ones Trump has received could prolong life for several weeks, or ultimately lead to recovery. Trump received his fifth and final dose of remdesivir, an FDA-authorized antiviral treatment, at the White House on Tuesday. Doctors said his dexamethasone treatment would continue there, as well.

"We all know if they're going to be effective, the drugs are more likely to be effective if given early on," David Battinelli, a professor of medicine at Hofstra, told Business Insider.

Galiatsatos said he frequently administers remdesivir to mitigate the impact of sepsis in ICU patients. Steroids like dexamethasone can also keep pneumonia from getting worse.

"We know this disease is very gradual and insidious," Galiatsatos said. "What we have become really good at is stopping the progression at some point, but that usually puts patients in kind of this horrible purgatory of still struggling for their lives, but not dying."

Patients who recover in the ICU are typically discharged after a month, Coffee said. From there, they should expect another month of rest, rehabilitation, and recovery, she added.

Even non-hospitalized patients may experience lingering symptoms, including fatigue and trouble breathing, for several months. Some may develop a chronic fatigue-like syndrome that persists for more than six months.

"Survival is just chapter one," Galiatsatos said. "The long-term effects we're all still learning in real time."

Aylin Woodward contributed reporting.