MYCIN is an early expert system developed in the 1970s at Stanford University to diagnose bacterial infections and recommend antibiotics. It uses a rule-based approach, where medical knowledge is encoded in the form of "if-then" rules. MYCIN was designed to assist physicians by providing recommendations based on patient data and symptoms. Despite its effectiveness in diagnosing infections, MYCIN was never used in clinical practice due to ethical and legal concerns regarding its recommendations. Nevertheless, it laid the groundwork for future developments in artificial intelligence and medical informatics, influencing later systems in the field of medical diagnosis.