Alison Stewart’s health crisis began on Feb. 22, a day jump-started by a 6 a.m. workout with her personal trainer. She was pushing herself to get back in shape after having donated a kidney to her sister six months earlier. The workout was routine, though she barely spoke, which was unlike her.

As the morning progressed, she began to feel confusion; she couldn’t engage in a lucid conversation or write a coherent text message. She headed into SoHo, to WNYC Studios, where she hosts the daily interview show “All of It,” and settled into her office to practice reading an introduction to a segment, but her phrasing hit roadblocks instead of flowing smoothly. Baffled, she sought out Kate Hinds, the show’s director.

Ms. Hinds was taken aback. “She looked very upset, her skin a little gray,” she recalled. “The disintegration was so stark and alarming. I was terrified.” She urged her to see a doctor.

Ms. Stewart’s doctor listened to her garbled words over the phone and told her to go straight to the emergency room. There, she was able to execute commands like touching her nose and walking backward and forward, signaling that she probably had not suffered a stroke, but a CT scan revealed a mass on her brain.

Ms. Stewart, 58, was soon talking gibberish, a dreadful and mystifying development for a woman who made her living by talking. Her decline was so rapid that her colleagues wondered whether she would ever be on the radio again.

“I knew she was trapped in there,” said Tracy Christian, one of several friends who had rushed to her side.