An outbreak is a sudden increase in the number of cases of disease that is not matched by normal expectations for the area and season. Outbreaks may affect a small and localized group or thousands of people across an entire continent. They may be caused by infectious diseases such as chickenpox, influenza, viral meningitis, bacterial infections (e.g., cholera), or environmental conditions such as pollution, poor hygiene and living conditions, and lack of safe water and sanitation.
Outbreaks can be identified through open-source syndromic surveillance systems that identify hot spots and patterns of unknown illness in low-income countries and where formal disease reporting capacity is limited. These systems are intended as an adjunct to and should not replace official disease reporting, but instead, are a way to identify early warning signals that are not being captured in the regular flow of information.
A syndromic outbreak is an event in which multiple persons who have similar symptoms are reported to the public health authorities at the same time, often associated with a common cause of illness. A syndromic outbreak case definition usually includes criteria such as person, place, and time of onset of the illness, and clinical features that distinguish the affected individuals from other cases of disease in the community. Place and time of onset are important as they allow for the exclusion of similar illnesses that are unrelated to the outbreak under investigation. This information can be used for planning and responding to outbreaks.