The problem with the medical sites isn't really that they're wrong, it's just that... well, some might call this paternalistic, but I think it's just professionalistic - I think I should tell you what you need to know, what's relevant. Otherwise, you just scare people. Now, if they need to be scared, or if I'm concerned, I should speak up, but otherwise, I think that should be considered "doing harm."
You see, those websites include EVERY possible cause of "headache." And they don't take into account anything other than what you typed in - "headache." So when a patient comes to me and says, "I know I have a sub-araknid hemmage because I have had a pretty bad throbbing headache for a week," it can be really hard to convince him that he does not in fact have a sub-arachnoid hemorrhage despite the fact that I went to school and learned about the signs and symptoms and they don't match at all.
Information misinterpreted and misused. Now that's dangerous.