It was then that I lost my breath, too. Suddenly I remembered what was coming: The musical is set in 1979 and the early 1980s, when a mysterious illness, whose origins were still unknown, began to claim lives by the dozens, then the hundreds and thousands, mainly gay men. I had actually seen “Falsettos” more than two decades ago, in Los Angeles shortly after it played Broadway. In the intervening years, I had forgotten the details of its plot, but in that instant I knew that this buoyant musical comedy was taking an inexorable turn toward heartbreak. There would be no happy ever after for Marvin and Whizzer.

The scene, acted with beautiful simplicity by Mr. Rannells and Mr. Borle, whose Marvin rushes to Whizzer’s aid, affected me as no other in the theater this year — as did the concluding, devastating scenes of the show — because it was like being thrust in a time capsule and brought back to the terrible years when the AIDS epidemic was at full force. I was living in Los Angeles in the late 1980s, in my 20s, and, as a gay man with many gay friends, existed in what can only be called a constant state of mortal anxiety: anxiety for my health but also that of so many people I knew, and many more that I didn’t.

It took me further back, too: to the moment in high school when I sat on the floor of the family room, reading a small article in The San Francisco Chronicle — in retrospect I’m sure it was by the Chronicle reporter Randy Shilts, who went on to write “And the Band Played On,” a definitive history of the early years of the epidemic — about that strange, unidentified illness. (Shilts himself died of AIDS, in 1994, at 42.)

Although I was not then “out,” I certainly knew I was gay, and as I absorbed the article (I can still see one of our Siamese cats sprawled out on the carpet in the sun beside me), I knew that in that moment the world had shifted around me. From then on, until who knew when, I would feel a shadow over my shoulder. The life ahead was suddenly not a simple question mark or a blank slate but a flashing alarm signal. (It didn’t help — or, in retrospect, perhaps it did — that I was already a world-class hypochondriac.)