If you are afraid of flying, then in your mind every time a plane flies safely from A to B increases the probability that next time it will crash. In reality, the opposite is the case.
If you are afraid of getting ill, then every day you wake up feeling well increases your anxiety that tomorrow you will wake up sick. Sick happens. A hypochondriac is someone primed and alert to spot the signs when they appear - or seem to appear.
Turn this around a bit and you could say that hypochondria is the fear of being well. The experience of wellness is spoilt by anxiety about the inevitable illness which is getting closer every day.
True, sick happens and, of course, everyone has to die some time. Whereas planes can be (and hopefully) are withdrawn from service before they get too old to fly safely, humans have little choice but to just go on until they die. Along the way, they will get ill. Very few people pre-empt their own decline and voluntarily withdraw themselves from service.
A bit more of the logic suggests that the hypochondriac's anxiety about getting ill will subside once they are actually ill. Once sick happens you have no need to get anxious about sick happening.
This may partly explain the paradox that older people generally express more satisfaction with life even though they have numerous ailments and more than occasional illnesses. Sick has happened and they can deal with it. The thing they couldn't deal with was the anxiety about getting ill.
At 65 I am excessively proud of the fact that I take no regular medication and really very little occasional medication. Most days I don't pop a pill, even an aspirin. That makes me unusual for my age group. But I have lots of little aches and pains and upsets and things going wrong ... In any one week there will be something not quite right. I have a suspicion that this is actually quite comforting. These little things are things I can deal with. They are not so bad after all. Of course, at the back of my mind I still succumb to the logic which says that the succession of these little things, like planes shuttling back and forth between A and B, must inevitably be followed by the lurking big thing, the crash.