At the beginning of the story the three friends, George, Harris and the narrator Jerome, are discussing how ill they feel. While George and Harris confess to feeling extremely giddy, Jerome confides that he is having liver problems. He knows this, he says, because he has just read an advert for a new liver pill describing various liver-related symptoms. Jerome concludes that he has all of these symptoms.
Jerome goes on:
It is a most extraordinary thing, but I never read a patent medicine advertisement without being impelled to the conclusion that I am suffering from the particular disease therein dealt with, in its most virulent form. The diagnosis seems in every case to correspond exactly with all the sensations that I have ever felt. (chapter 1)
Jerome here is remarking humorously on his own hypochondriacal tendencies. He is never really ill but he is always worrying that he is ill, and is too easily influenced by descriptions of illnesses; he ends up thinking that he has the symptoms of any malaise that he happens to read about. He goes on to illustrate this in a highly amusing fashion, by relating how once he read a medical tome in the British Museum and became convinced he had practically every illness known to man.
However, Jerome is not just having a dig at himself. He is also wryly commenting on the modern trend of advertising illnesses, their symptoms and cures, all over the place. He observes that sometimes the ‘old-fashioned’ cures, such as just getting on with things and not thinking about being ill, work better than ‘all the dispensary stuff’ (chapter 1)