An index case is the patient that serves as the central point of reference in a disease outbreak. This may be the initial case identified in an outbreak. A synonym may be patient zero. The first generation of cases from the index case are primary cases. Cases that arise from the primary cases are considered secondary cases. The concepts of an index case, proband, and informant are similar but arise from different disciplines. Babbage has a great description of informants in sociological research. (Babbage 2007) The index case is typically used by disease outbreak (usually an infectious disease) investigators, epidemiologists, physicians, and communicable disease nurses to describe the case that is noticed first in a case series, outbreak or epidemic. The proband is used in genetics to describe the case that seeks treatment first. The proband case typically is a disease that has a genetic basis. In sociological research conducted in the field an informant, is sometimes

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