Hypochondria is a word that has changed in meaning over its history, as I discovered when I went to talk given by Dr Jane Darcy of King's College on melancholia in the 18th century.
Hypochondria comes from ancient Greek and literally refers to the soft part of the body below the ribs. In the 17th and 18th centuries hypochondria was considered an actual disease based around disorders of the stomach or digestive system. Boswell referred to Samuel Johnson as a 'hypochondriack' (sic), although Boswell was renowned for being one himself and even used the pseudonym 'The Hypochondriack' when writing articles. At the time the adjectives hyp, hypo, hypt and hypped were also in use to describe a hypochondriac, and the OED has a related entry 'hipped' which it describes as colloquial British English meaning "affected with hypochondria; morbidly depressed or low-spirited".
In the 18th and 19th centuries hypochondria was thought mainly to affect men, and it was a sort of male equivalent to hysteria (thought to affect only females, which is why the origin of the word is the Greek word for 'womb'). It was only in the late 19th century that hypochondria gained the more pejorative meaning it has today ie imagining oneself to be ill.