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Last Edited: 11/9/06

Disease Clusters

A disease cluster is the occurrence of more than the expected number of people diagnosed with a certain disease within a specific group, a geographic area or a period of time.  Disease clusters may be suspected when people report that several family members, friends, neighbors, coworkers or community members have been diagnosed with the same illness.  Clusters of various diseases have concerned scientists for centuries.  Some recent disease clusters include the initial cases of a rare pneumonia among homosexual men in the early 1980s that led to the identification of the human immunodeficiency virus (HIV) and acquired immunodeficiency syndrome (AIDS).  Another, very recent disease cluster, was the 2003 outbreak of a respiratory illness, later identified as severe acute respiratory syndrome (SARS), caused by a previously unrecognized virus.

 

Investigating disease clusters of any kind is the mission of epidemiologists (scientists who study the frequency, distribution, causes and control of diseases in populations).  With their knowledge of diseases, environmental science and statistics, epidemiologists try to distinguish actual disease excesses from excesses that are due only to chance.

 

There are several challenges in investigating disease clusters.  Epidemiologists must make sure that the suspected cluster they are investigating involves one disease (not many).  Epidemiologists must determine that a suspected exposure could have actually triggered disease outbreak, based on what is known about the likely causes.  Next, they must show that the number of observed cases is significantly greater than what they would expect.  Then they must determine if the cluster could have occurred by chance.

 

At the request of concerned community members or local County Health Officers, the Environmental Health Investigations Branch (EHIB) has examined several suspected disease clusters occurring in California.  The Branch approaches such investigations with serious concern but also with caution and care to avoid the pitfalls inherent in such studies.

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