WASHINGTON — The lights were low and the disco balls spinning as a cake with a fiery sparkler shooting flames into the air was brought out to a robust rendition of “Happy Birthday,” joined by President Trump. The birthday girl, Kimberly Guilfoyle, the girlfriend of Donald Trump Jr., then pumped her fist in the air and called out, “Four more years!”

It was a lavish, festive, carefree Saturday evening at Mar-a-Lago a week ago in what in hindsight now seems like a last hurrah for the end of one era and the beginning of another. In the days since then, the presidential estate in Florida has become something of a coronavirus hot zone. A growing number of Mar-a-Lago guests from last weekend have said they are infected or put themselves into quarantine.

A week later, the White House physician announced on Saturday night that the president had tested negative for the virus, ending a drama that played out for days as Mr. Trump refused repeatedly even to find out whether he had contracted it after exposure to multiple infected people. The result came less than 24 hours after the White House put out a misleading midnight statement saying there was no need for such a test at roughly the same time the president by his own account was actually undergoing one in deference to public pressure.

But either way, the Mar-a-Lago petri dish has become a kind of metaphor for the perils of group gatherings in the age of coronavirus, demonstrating how quickly and silently the virus can spread. No one is necessarily safe from encountering it, not senators or diplomats or even the most powerful person on the planet seemingly secure in a veritable fortress surrounded by Secret Service agents.