Actually it does. It's fed with data that says: Condition X is usually caused by Y with Z% probability, etc etc.

Then you feed it all the symptoms, sometimes it asks questions, and gives you a list of causes sorted by probability.

It works almost exactly like this game http://www.guessthename.com/

Apparently the biggest problem with it is that doctors won't use it.

The second problem is feeding data - you basically have to parse every medical paper ever published.

Wait a second, if the system does in fact exist, and it works well, and the biggest problem is that doctors won't use it, I'm with amichail.

Guessing a name is a much simpler problem than guessing a disease. Can you point to any such systems for medical diagnoses?

Here's a paper on one (just a random one from google) http://www.med.univ-rennes1.fr/cerf/publi/ADM_index1.html

Here's a patent: http://www.freepatentsonline.com/6607482.html

There are lots of a papers, and a small number of working prototypes, but I have not heard of any that are actually being used.