Agreed. I really wish that there was a site like webMD that actually included rates of the diseases and the symptoms. I don't think it would be a big step to go from there to something that would actually propose cost-effective tests for you based on your symptoms.

e.g. You select sore-throat and fever as symptoms and it says that out of people with those symptoms, 70% have a cold, 25% have a strep infection and 5% have something else (these numbers are completely made up). An even better system would then look at which tests you could do to better nail down the probabilities, which could be as simple as asking some questions like "Do you have any visible rashes?" or asking for test results like a quick strep test.