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An outbreak is a sudden rise in the occurrence (the number of cases) of a disease. Some outbreaks are expected each year, like influenza, hand-foot-and-mouth disease, and bronchiolitis. This term might be used when a single case of an unusual disease occurs. In 2006, a team of researchers, epidemiologists and informaticians at Children's Hospital Boston developed HealthMap, an online tool for disease outbreak monitoring and real-time surveillance of emerging public health threats.

The definition of an influenza pandemic given by the World Health Organization (WHO) states that “an influenza pandemic occurs when a new influenza virus appears against which the human population has no immunity, resulting in several simultaneous epidemics worldwide with enormous numbers of deaths and illness”.
Such a definition has been changed in 2009, in concomitance with the H1N1 influenza outbreak, and lost part of its severe meaning (see also Swine flu).
The occurrence of pandemics is rare, and are different compared to regular seasonal epidemics of influenza. The most severe influenza pandemic in recent history was the 1918 Spanish flu, which is estimated being responsible for the deaths of over 50 million people.