An emerging infectious isease (EID) is an infectious disease whose incidence has increased in the past 20 years and threatens to increase in the near future. EIDs include diseases caused by a newly identified microorganism or newly identified strain of a known microorganism (e.g. SARS, AIDS); new infections resulting from change or evolution of an existing organism (e.g. influenza), a known infection which spreads to a new geographic area or population (e.g. West Nile virus), newly recognized infection in an area undergoing ecologic transformation, and pre-existing and recognized infections reemerging due to drug resistance of their agent or to a breakdown in public health (e.g. tuberculosis).