A disease outbreak occurs when the number of cases of a disease exceeds what is expected. This may be due to a variety of factors, such as a difference in how cases are reported or sudden changes in population numbers. The number of cases can be a critical factor in whether a disease is considered to be at public health risk.
Viruses can be spread by touch (like a hunter touching an infected animal), breathing in droplets when someone coughs, or eating contaminated food. Disease can also be spread through water, such as when flooding disasters contaminate local drinking water supplies. Lastly, diseases can also be spread by environmental factors such as insect breeding sites or airborne contamination of a pathogen.
Once a disease is spreading from person to person, it can reach epidemic or pandemic proportions. This is when the disease infects a large number of people in one or more communities and is spreading rapidly.
To determine if there is an outbreak, health departments analyze data from sources like a disease registries and hospital discharge records to see whether the current number of cases is above or below the expected number of cases for a particular period. A case definition is then created, which includes criteria for person, place and time (e.g. X nursing home, Y high school) to identify cases that are related. If an outbreak is identified, the people involved are typically required to be notifiable to their local health department.
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