He returned to Marseille and contacted Jean Gemähling, a Resistance fighter and member of Mr. Fry’s network. On his advice, Mr. Rosenberg went to Grenoble. There, in August 1942, he was rounded up with several hundred other Jews, most of them foreign-born. He was taken by bus to a transit camp, Vénissieux, outside the city of Lyon. It was blisteringly hot; he slept on the ground, outside the barracks. On his first night he was shaken awake by a soldier who told him the camp inmates would soon be sent to a labor camp in Poland.

In the morning he ran into a fellow internee, a medical student who was the sister of a friend. He asked her what illness might help him avoid the transport. She suggested peritonitis and told him the symptoms. He fell to the ground — his acting skills in use — and began writhing and moaning. In the camp infirmary he rubbed a thermometer until it registered a dangerous fever. The ruse worked: He was rushed to a nearby hospital and given ether. “When I woke up, I really had pain in my side. I touched it and it had a big bandage.” His appendix had been removed.

When he next woke, a young French nurse sat beside him. She told him that patients from the camp would be sent on the next transport. “I said, ‘Can you do me a favor?’” He asked for paper and pen and implored her to mail a note.

“Have I mentioned it to you yet?” asked Dr. Rosenberg, picking up the narrative the next day as he drove from Bard’s campus to his home in Rhinebeck. It is clear he relishes this story. Within days, he said, a priest connected to the Resistance arrived. “The plan was a simple one,” he continued, recounting how the letter found its way to the network of Abbé Alexandre Glasberg, a priest who had converted from Judaism. Father Glasberg ran an operation of Christians committed to rescuing Jews from the dragnet.

“Keep in mind they had taken my clothes away,” Dr. Rosenberg said. A sympathetic priest would arrange for a parcel to be hidden in the hospital toilet containing a shirt, a pair of shorts and sandals. Below the window there would be a bicycle. He still recalls the priest’s parting words. “‘Good luck. May God bless you,’” Dr. Rosenberg quoted.

Barely healed, he set off from the hospital by bicycle, making his way to a safe house where he could recuperate. There, he received a new identity, Jean-Paul Guiton — a “fine French name” — whose father was dead and whose mother was Alsatian (to account for his accent). His birthplace, he said, was now Saint-Malo, a city whose town hall, and birth records, had been destroyed.