A disease outbreak occurs when more cases of a certain illness are observed than expected in a given community. This community can be as small as a school or restaurant or as large as an entire nation or continent. The underlying cause of the illness may be transmitted from person to person, or it can be spread via the environment or food.
Infections can be caused by bacteria, viruses, parasites, or even chemical substances such as mercury (Minamata disease). The pathogens themselves are often zoonotic; for example, malaria is transmitted by the bite of an infected mosquito. The disease can be endemic, epidemic, or pandemic.
Regardless of the type of outbreak, the most common underlying cause is poor water and food quality or inadequate sanitation. Other causes include contaminated sewage, improper waste disposal, poor ventilation, or unclean hands. People are a natural reservoir of some infectious diseases, such as influenza.
The most important part of the investigation is establishing the underlying source of the disease. This can be difficult, especially if it is a new or emerging disease with few case histories. The identification of the underlying cause requires careful analysis of the data to be able to distinguish between the true number of outbreaks and other factors such as increased reporting, improved diagnostic procedures, or screening campaigns (detection bias).
Once the underlying cause is identified, measures can be taken to prevent further spread. These include calling back contaminated products, closing a manufacturing plant, disinfection, improving hygiene, removing infected persons from the source area, and health education and information campaigns.