There's a radio commercial that's been out lately. I don't even know what it's a commercial for--some hospital or insurance plan, maybe? In any case, one of the characters in the commercial is supposed to be a hypochondriac worried about the illnesses he might have. And he's just randomly spewing out the names of various conditions that don't even have similar symptoms.
But there is logic even to hypochondria.
In my experience, exaggerated anxiety over illness tends to evolve in
one of two ways. In the first way, a symptom--say, a headache or
stomach-ache or leg pain--will lead the person to research the various
illnesses associated with that symptom. Therefore, the person with the
stomach-ache might worry about an ulcer or appendicitis, but he would
not imagine that he had a skull fracture or a detached retina. In the
other manifestation, a hypochondriac might hear about a particular
dangerous illness, especially one with very vague or common symptoms,
and might start asking herself whether she has those symptoms ("Is my
vision a little blurry? Haven't I been feeling tired lately?")
I have never met a person who displayed hypochondria by just flinging
the names of unrelated diseases into a conversation, willy-nilly. This
fictional character rings false, a victim of insufficient research or
insufficient imagination. In a commercial, that may not matter as much,
because I think the suspension-of-disbelief goals of a commercial are
different from those of a novel. But most novels do require that
suspension of disbelief. Characters should always make sense to themselves, even if they seem strange or extreme to the other characters.