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.