Medical experts said that people who have had a liver transplant often suffer from a variety of medical problems that are not life-threatening.

A person with knowledge of the situation said that Mr. Jobs suffers from immune system issues common with people who have received liver transplants and that, as a result, his health suffers from frequent “ups and downs.”

In recent weeks, Mr. Jobs began a down cycle and slowed his activities at Apple, said the person, who refused to be identified because he was not authorized to discuss Mr. Jobs’s condition. Mr. Jobs has been coming to the office about two days a week and has appeared increasingly emaciated, the person said. He has frequently had lunch in his office, rather than in the company cafeteria, the person said.