this gross example of irrationality

soren, please don't take this the wrong way, but based on what I've seen you post so far, you are not a strong enough rationalist to say things like this yet. You are using your existing knowledge of biases to justify your other biases, and this is dangerous.

Doctors have a limited amount of time and other resources. Any time and other resources they put into considering the possibility that a patient has a rare disease is time and other resources they can't put into treating their other patients with common diseases. In the absence of a certain threshold of evidence suggesting it's time to consider a rare disease (with a large space of possible rare diseases, most of the work you need to do goes into getting enough evidence to bring a given rare disease to your attention at all), it is absolutely completely rational to assume that patients have common diseases in general. .