There are two great U.S. government websites that provide information about diseases, illnesses, syndromes and health conditions. Entries in each resource include information on symptoms, diagnosis, treatment, prevention and risk factors.
Diseases & Conditions (CDC): A-Z list of diseases, disorders, and conditions. Maintained by the Centers for Disease Control and Prevention
Health Topics (MedlinePlus): Search for a disease by browsing an A-Z list, by body system, or by broad disease type. Maintained by the National Library of Medicine