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President Donald Trump thanked Walter Reed National Military Medical Center staff and even cracked a joke about his age as he fights COVID-19.

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President Donald Trump’s battle with COVID-19 will come to a critical turning point in the next few days as the disease tests his immune system.

On Saturday, Trump and his doctors acknowledged the importance of the coming days as the illness enters what White House physician Dr. Sean Conley called “phase 2.”

In a video statement released Saturday evening from Walter Reed National Military Medical Center, Trump echoed the concern: “I’m starting to feel good. You don’t know over the next period of a few days, I guess that’s the real test, so we’ll be seeing what happens over those next couple of days.”

The course of COVID-19 can be highly variable, but the next three to five days are likely to be crucial, physicians who have treated hundreds of coronavirus patients told USA TODAY.

Several days after symptoms of COVID-19 appear, the body’s immune system must make an important switch to fight the virus with precision — or possibly face life-threatening consequences.

Watch: Supporters outside hospital where Trump is staying

Watch: Dr. Sean Conley did not answer several questions regarding Trump receiving oxygen

COVID-19 patients can “look pretty good for a few days, then they go south,” said Dr. William Schaffner, an infectious disease specialist at Vanderbilt University.

That rapid deterioration can occur when the body’s immune system, unable to successfully target the virus, causes widespread collateral damage as it “brings in the troops,” Schaffner said.

A typical timeframe for patents’ decline is about five to 10 days after the person starts getting sick, said Dr. J. Randall Curtis, a professor of pulmonary and critical care at the University of Washington school of medicine in Seattle. 

Conley on Saturday said Trump is in his third day of fighting the virus.

During the early part of a patient’s COVID-19 illness, the body uses an “agnostic” immune response, said Dr. Greg Poland, director and founder of Mayo Clinic Vaccine Research Group. It doesn’t know what it’s fighting, but realizes something potentially dangerous is occurring. That’s called the innate immune system.

Key to a successful recovery is an immune response that targets the coronavirus itself. That’s called the adaptive immune system. 

To avoid serious illness, a patient’s innate and adaptive immune systems must stay in balance, and the virus itself must not cause serious complications along the way.

Age is a risk factor. Older patients tend to be less successful in activating the adaptive response, according to Melissa Nolan, an infectious disease expert and professor at the University of South Carolina.

Trump turned 74 in June, putting him at 90-times higher risk of death than someone in their 20s, according to data from the U.S. Centers for Disease Control and Prevention.

But the course of COVID-19 can be highly variable. The president’s VIP medical treatment and access to cutting-edge therapies make the trajectory of the