The nation's top infectious disease expert said he tested negative for coronavirus Saturday when asked why he wasn't wearing a face mask as the pandemic spreads across the nation.

"There are a couple of reasons. One of them is part of the, in fact the major reason to wear a face mask is to protect you from infecting you," Dr. Anthony Fauci said Sunday at a news conference with the White House coronavirus task force. "I had my test yesterday and it's negative."

President Donald Trump added: "very good answer."

Fauci's comment comes after Trump announced last week that he had tested negative for coronavirus a second time, highlighting the unique challenge facing the White House as officials grapple with both how to contain the virus and take steps to avoid spreading it in the executive branch and task force.

Fauci, who serves on the task force, warned Americans in an interview on CBS earlier Sunday that "it is going to be a bad week" ahead as there is an escalation in cases, but that "within a week" or so, the number of cases should start to flatten out.

"Are you saying, doctor, despite the deaths that we may see, that mitigation is working, and that you do have this outbreak under control?" CBS' Margaret Brennan asked Fauci.

"I will not say we have it under control, Margaret. That would be a false statement. We are struggling to get it under control, and that's the issue that's at hand right now," Fauci replied.

Acknowledging Trump's earlier comments about "starting to see the light at the end of the tunnel" of the outbreak and recent reports that the upcoming weeks would be grim, Fauci also said during Sunday's briefing that the difference between the two characterizations "seems to be inherently contradictory, but it really isn't."

"If we start seeing now a flattening or a stabilization of cases, what you're hearing about potential light at the end of the tunnel doesn't take away from the fact that tomorrow or the next day is going to look really bad," Fauci continued. "So we've got to make sure we realize we're always talking about a two-and-a-half week lag."

"It's really not incompatible with what we're saying," he continued.

Fauci also reiterated experts' pleas for the public to continue using basic strategies to prevent the virus' spread, asserting that "the only tool, but the best tool we have is mitigation."

"So, if we really want to make sure that we don't have these kinds of rebounds that we're worried about, it's mitigation, mitigation, mitigation," he added.