The section of the story that this question is referring to can be found in the early parts of chapter one. J, the narrator, begins this chapter by telling readers that he, George, and Harris were all sitting around talking about their general poor health.
We were sitting in my room, smoking, and talking about how bad we were—bad from a medical point of view I mean, of course.
I'm sure the smoking has something to do with their general lack of feeling great.
J. then specifically tells readers that his liver was "out of order." He was recently able to read up on a pill that fixes liver issues, and he states that he had each and every symptom listed for a bad liver.
I knew it was my liver that was out of order, because I had just been reading a patent liver-pill circular, in which were detailed the various symptoms by which a man could tell when his liver was out of order. I had them all.
The very next paragraph raises reader suspicions about J. and his health. He tells readers that whenever he reads about a particular disorder, he can't help but feel that everything regarding that particular disorder applies exactly to him. It's at this moment that readers can seriously suspect that J. is a hypochondriac.
Next, J. tells readers about a trip that he made to the British Museum "to read up on the treatment for some slight ailment" that he believed that he had. In this particular case, J. believed that he had hay fever. He read up on hay fever, and his suspicions were confirmed. Unfortunately for J., he began to peruse through the rest of the medical book. He read about disease after disease, and he became convinced that he exhibited the signs and symptoms for each disease.
J. believed that he must be a medical marvel, and he reported to his doctor to share the news. The only malady J. did not believe he suffered from was "housemaid's knee."
"Everything else, however, I have got."
The doctor examined J. and wrote him a prescription for a better diet, more exercise, and less reading about diseases. J. followed the directions, and he miraculously became healthy again.
I followed the directions, with the happy result—speaking for myself—that my life was preserved, and is still going on.