When an outbreak of an infectious disease occurs, it usually arises when the disease is introduced into a susceptible population.
The first person with the disease - who may be the person who introduced the disease in the community, or may simply be the first case to be identified - may be referred to as the index case. The term is also used in clinical genetics and construction of family trees. Proband or propositus are alternative words of the same effective meaning.
By extension there may be reference to the index family etc.
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