When the computer "meets" a patient, it asks a series of questions to help it follow the rules it has been given, until it has enough information to make a "diagnosis".
Expert systems will only use the rules they have been given to reach a conclusion. They are not usually able to adapt themselves.
They rely on the quality of the information and rules that they are given. If a rule or information is incorrect, the system ends by producing an incorrect diagnosis - this could be fatal!
Computers have no common sense!