An outbreak is a disproportionate number of cases of a disease in a defined community, area or season. This is often due to the presence of an infectious agent – either directly from person to person or via insect vectors, from animal reservoirs, or from environmental sources such as food and water. The number of cases in an outbreak varies according to the size and type of the population that has been exposed, and the number of people at risk. Outbreaks can occur in all regions of the world and may affect thousands of individuals across a continent.
Epidemic-prone diseases such as diarrhoeal illnesses, influenza, mumps, measles and pneumonia are common causes of disease outbreaks. Other diseases prone to epidemics include zoonotic infections (infection transferred from animals to humans), airborne and waterborne pathogens, chemical poisoning (such as the poisoning of people with mercury in Minamata, Japan) and radioactive contamination (from nuclear power plants or from accidental releases).
The purpose of epidemiological investigations into disease outbreaks is to detect and characterize the cause, source and modes of transmission of the pathogen. This requires simultaneous application of epidemiological, microbiological, toxicological and clinical methods, including the generation of hypotheses and their formal testing.
Since Edward Jenner and John Snow observed the devastating epidemic of smallpox in England in the 18th century, and based on this developed a vaccine for it, outbreak investigation has been an integral part of public health practice. The investigations of outbreaks allow for the development of public health regulations and prevention guidelines. It is therefore important that governments, communities and families take preventive measures such as regular hand washing, covering sneezes and coughing, isolation of ill persons and taking care of safe drinking water and food.
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