Fox News medical contributor and doctor Marc Siegel on Friday knocked President Trump Donald John TrumpBubba Wallace to be driver of Michael Jordan, Denny Hamlin NASCAR team Graham: GOP will confirm Trump's Supreme Court nominee before the election Southwest Airlines, unions call for six-month extension of government aid MORE’s controversial comments about disinfectant possibly being used to treat coronavirus patients, saying “the president is not a physician.”

The doctor appeared on "America's Newsroom" and said he thought Trump was referring to the virus being able to be easily killed on surfaces with disinfectant, soap and water, noting the increased need for proper hand-washing.

“There’s no evidence that ingesting anything into your body is going to deal with it and I’ll tell you why,” Siegel said. “Once the virus is in the cells, it’s spreading from cell to cell. You’re not going to be able to kill it that way. That’s why we’re looking for systemic treatments.”

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Siegel noted that the vast majority of coronavirus patients have a “mild case.”

“The real way to fight a virus off is to be in good health, to sleep well, to eat well, to exercise,” the contributor said. “A lot of that stuff we’re having trouble doing right now because we’re under such stress. But no, there’s not a disinfectant you can introduce into the body to fight this. By the time the virus gets in, it’s already beyond that.”

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Trump sparked criticism from medical professionals for the remarks he made during Thursday's White House briefing.

While frequently turning toward the administration officials in the briefing room, Trump suggested medical experts should study exposing the human body to heat and light as a treatment for coronavirus.

He also asked if there was a way to use disinfectants on the body "by injection inside or almost a cleaning."

"Maybe you can, maybe you can’t ... I’m not a doctor. But I’m, like, a person that has a good you-know-what," Trump said, pointing to his head.

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Trump, however, on Friday walked back his remarks and said he was being sarcastic.

"I was asking a question sarcastically to reporters like you just to see what would happen," Trump told reporters at an Oval Office bill signing.

"I was asking in a very sarcastic question to the reporters in the room about disinfectant on the inside," the president continued. "But it does kill it, and it would kill it on the hands and that would make things much better. That was done in the form of a sarcastic question to the reporters."