PRESIDENT TRUMP has joined the ranks of millions of Americans who tested positive for the coronavirus. On Friday, he was taken to Walter Reed National Military Medical Center, where he is expected to remain for several days. We wish a speedy and uneventful recovery to him, first lady Melania Trump, White House adviser Hope Hicks and anyone else around them who contracted the virus.

Whether his recuperation is speedy or slow, however, the president’s state of health will be a matter of urgent public concern. Americans must know whether Mr. Trump is well enough to make decisions of vast importance, including those regarding the nation’s security. Experts must dispassionately trace the source of the outbreak to discern who else must take precautions. The public must be reassured that senior staff across the White House, federal agencies and Congress will be tested regularly, watched and, when necessary, isolated to contain any spread among the country’s senior leadership. That also includes Democratic presidential nominee Joe Biden and others who attended Tuesday’s debate in Cleveland; though the former vice president tested negative on Friday, the incubation period for the coronavirus can extend to 14 days.

As Mr. Trump awaited confirmation that he was ill on Thursday night, he blamed Ms. Hicks’s infection on members of the military and police officers who want to “hug” and “kiss” him and his staff. On the other hand, Notre Dame University President Rev. John I. Jenkins revealed Friday that he tested positive after he attended a Saturday White House Rose Garden ceremony for Judge Amy Coney Barrett, suggesting another possible viral path. Like most other attendees — including Sen. Mike Lee (R-Utah), who also announced Friday he had tested positive — Mr. Jenkins did not wear a mask and sat in close quarters with others. If the actual source of Mr. Trump’s infection can be identified, it should be fully disclosed: It will inform public health officials on where they should look for new infections and show Americans how the virus spreads.

The looming election may supercharge Mr. Trump’s instinct to hide or twist the truth. The record is already discouraging. The White House learned that Ms. Hicks had tested positive before Mr. Trump flew to an indoor roundtable fundraiser on Thursday. Yet the president still held the fundraiser, without wearing a mask, as though nothing were amiss. It was not the White House but a Bloomberg News report that eventually informed the public that a close presidential aide was sick with an extremely contagious virus.

White House Chief of Staff Mark Meadows assured the country on Friday that Mr. Trump was experiencing “mild symptoms.” But such reports from political aides are not adequate. Mr. Trump was being examined late Friday by doctors at Walter Reed, where presidents go for checkups and treatments. The results should be fully disclosed by those doctors, who should answer reporters’ questions at news conferences.

Mr. Trump is in the same predicament that so many others have struggled through since February, and far too many Americans can speak to the toll it can exact. The president may have thought he was safe with intensive testing but lax mask-wearing. His diagnosis re-proves that everyone is vulnerable to this plague. People must wear masks and socially distance. We can hope only that Mr. Trump’s misfortune will drive that point home to his followers, and all Americans.

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