You're right. You have to update the probability with all the knowledge you have. Although I would say self diagnosis has a very, very high false positive rate, and if you know you're a hypochondriac that would make the probability drop, plus we don't even have a test for this that is 90% accurate. I'm not sure the difference is an order of magnitude :)

I think my point stands that it doesn't make sense to test for extremely rare diseases because of a self diagnosis.