Infectious diseases may be caused by viruses, bacteria, fungi, or protozoan pathogens and may or may not be contagious. Some infectious diseases are prolonged as a result of reactivation of the immune system through antigenic variation. In some cases the pathogen may continue this process until it overwhelms and exhausts the immune system.

• WHO International Classification of Diseases
• National Center for Infectious Diseases
• HEALTHmap - global disease alert map