The basic definition of an emerging or re-emerging infectious disease is a disease whose incidence has increased in a defined time period and location. If the disease was unknown in the location before, the disease is considered to be emerging. However, if the disease had been present at the location in the past and was considered eradicated or controlled, the disease is considered to be re-emerging. Diseases considered to be emerging or re-emerging include avian influenza, West Nile virus, bovine tuberculosis in wildlife, and Lyme Disease.

Recognition of an emerging disease can occur because the disease is present in the population for the first time, because the disease has been detected for the first time, or because links between an infectious agent and a chronic disease or a syndrome have only recently been identified. Many of these emerging diseases are zoonotic, and rely on animal populations as reservoirs of infection.

Most emerging infections are caused by pathogens already present in the environment, brought out of obscurity or given a selective advantage by changing conditions and afforded an opportunity to infect new host populations. These changes include ecological changes, such as those due to human activities or to anomalies in climate; demographic changes and behavior; travel and commerce; technology and industry; microbial adaptation and change; and breakdown of public health measures. Many factors precipitate emergence by placing humans or animals in contact with a natural reservoir or host for an infection unfamiliar but already present (often a zoonotic or arthropod-borne infection), either by increasing proximity or, often, also by changing conditions so as to favor an increased population of the microbe or its natural host.