Introduction: One health and emerging infectious diseases.

Those infectious diseases that have increased in the past two decades or that threaten to emerge are considered emerging infectious diseases. Many of these diseases are acquired through contact with plants, insects and animals. "One health" acknowledges the interconnectedness of environment, plant, animal and human health. The introduction to this issue of Seminars in Diagnostic Pathology gives the definition of emerging and reemerging infectious diseases, explores the concept of "One Health", explains the evolution of testing for infectious agents as it relates to emerging infections, and considers the difficulties in predicting which and where the next emerging infection will occur.