The delivery driver did not want to go upstairs.

The driver, Zhang Sai, hovered outside an apartment building in Wuhan, the central Chinese city at the heart of the coronavirus outbreak. He had been ordered not to take food to customers’ doors in order to minimize the risk of infection.

But the woman on the phone was pleading, he recalled. The food was for her mother, who couldn’t go down to meet him.

Mr. Zhang relented. He would drop off the order and sprint away. As he placed the bag on the floor, Mr. Zhang said, the door opened. Startled, he rushed away. Without thinking, he said, jabbed the elevator button with his finger, touching a surface he feared could transmit the virus.

That was how Mr. Zhang, 32, found himself speeding back to his delivery station with one finger held aloft, careful not to touch the rest of his hand — a quarantine in miniature.