Why were the narrator and his friends feeling out of sorts?

Each of the men believed that they were suffering from sort of malady..... they were hypochondriacs.

We were all feeling seedy, and we were getting quite nervous about it.  Harris said he felt such extraordinary fits of giddiness come over him at times, that he hardly knew what he was doing; and then George said that he had fits of giddiness too, and hardly knew what he was doing.  With me, it was my liver that was out of order.  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.