Robert C. O’Brien, President Trump’s national security adviser, has tested positive for the novel coronavirus, the White House said Monday as the United States recorded more than 26,000 new cases of the infection that is coursing out of control.

O’Brien is the highest-ranking Trump administration official known to have tested positive for the virus. Trump said Monday that he had not seen O’Brien recently, but the White House did not respond to questions about their interactions, whether other White House employees might have been exposed or the timing of O’Brien’s illness.

“He has mild symptoms and has been self-isolating and working from a secure location off site,” the White House said in a statement about O’Brien. “There is no risk of exposure to the President or the Vice President. The work of the National Security Council continues uninterrupted.”

The White House did not respond to questions about how O’Brien’s illness was discovered. For several weeks, the White House has been routinely administering rapid-result tests for the coronavirus to most people who will be in proximity to Trump.

O’Brien has been working regularly out of his White House office and most NSC staff members and other White House staffers are also working in their offices during the pandemic, in contrast with many businesses and private employers.

“I haven’t seen him lately,” Trump said as he left the White House for a trip to North Carolina, where the president toured a biotech lab involved in the research effort toward a coronavirus vaccine.

“I heard he tested, yeah,” Trump said, adding that he planned to call O’Brien later Monday.

The president was not wearing a mask as he spoke and did not wear one later in the day at the Fujifilm Diosynth Biotechnologies facility in Morrisville, N.C.

Federal health officials recommend face coverings as a front-line defense against the spread of the virus that has shuttered schools and businesses and altered much of daily life. Trump has said that he will wear a mask when necessary but that he does not need to do so when those around him have all been tested.

Trump has said he is tested daily, while press secretary Kayleigh McEnany said last week that Trump is sometimes tested more than once a day. The president later said he had not been tested more than once on a given day.

Following a tour of the facility, Trump likened the push for a vaccine to a wartime effort, predicted a quick economic rebound from the pandemic and repeated misleading statistics about the number of tests conducted in the United States. He also said his reelection campaign shows that he is leading in opinion polls, the opposite of findings in independent national polls.

“We need all Americans to be conscious about their actions and to exercise extreme vigilance,” Trump said. “I trust all Americans to do the right thing, but we strongly advise everyone to especially, especially, focus on maintaining a social distance, maintaining rigorous hygiene, avoid large gatherings and crowded indoor bars and wear masks when appropriate.”

O’Brien is in frequent contact with Trump and is known to be in the inner circle of his advisers. They last appeared publicly together during a visit to the U.S. Southern Command in Florida on July 10, before O’Brien left for a trip to Europe that spanned several days.

O’Brien has told friends that he thinks he became infected at a family gathering roughly a week ago, one person familiar with his case said. The person spoke on the condition of anonymity because he was not authorized to discuss the case on the record.

White House economic adviser Larry Kudlow told reporters at the White House on Monday that he believes that O’Brien’s daughter also had contracted the virus and that O’Brien could have been infected by her.

“I think it’s a light case,” Kudlow said. “I wish him well. . . . He’s a fabulous colleague and a brilliant policymaker.”

One of Trump’s military valets and Vice President Pence’s press secretary have previously tested positive for the virus.

O’Brien, appointed to his role in September, was named to the White House coronavirus task force when it was launched in January.

O’Brien’s tenure as national security adviser has been calmer than the incendiary one of his predecessor, John Bolton. A foreign policy hawk, especially in regard to China, O’Brien was among the first administration officials to squarely blame that country for failing to stop the virus from spreading beyond its borders.

Speaking at a foreign policy forum on Feb. 5, O’Brien said China hid the extent of the outbreak in its early weeks and cost the rest of the world precious weeks to prepare. At that time, Trump was still saying that the virus was not a threat to the United States and that Chinese President Xi Jinping was doing a good job containing the outbreak.

O’Brien’s positive test was first reported by Bloomberg News.