Emerging infectious diseases are caused by old, new, and mutant microorganisms. Emergence of these pathogens can be attributed to changes in the characteristics and risk factors of patients, the widespread use of antibiotics, changes in the environment, the role of xenotransplantation, and international travel. In the United States, the incidences of C. difficile, cyclosporiasis, enterohemorrhagic E. coli gastroenteritis, Hantavirus, hepatitis C virus infection, and
Lyme disease have increased significantly over the past two decades. Malassezia pachydermatis, extended spectrum beta lactamase (ESBL), Gram negative bacilli, and antibiotic resistant Enterococci, S. aureus, S. pneumoniae, and M. tuberculosis have also emerged prominently. Although not yet seen in the United States, variant Creutzfeldt-Jakob
disease has made a great emotional impact on this country. Identifying, treating, and controlling emerging infectious
disease and pathogens have created enormous challenges.