index case

index case, in public health, the first case of a disease or other condition that is noticed by health authorities. Index cases may be reported in relation to infectious disease outbreaks or to occurrences of noninfectious diseases or health conditions. They may also be reported within a region or within a group, such as a population or a household. Knowledge of the index case is especially useful for determining the way in a which a disease or other health condition is transmitted. In instances of infectious disease, the index case further signals the existence of an outbreak and may provide insight into disease reservoirs (where the disease resides between outbreaks).

The index case is sometimes also referred to by the term patient zero. The latter, however, is a less precise term: it may be used to describe either the first case of the disease, the first case on record, or the first case within a specific group. Likewise, there is often confusion between index case and primary case, which is used specifically in reference to the individual who introduces an infectious disease into a group when the disease is spread from person to person. In some outbreaks, the index case and the primary case may refer to the same instance of reported disease; however, in many disease outbreaks, the primary case is unknown. By contrast, for any outbreak of disease that is noticed within a region or group, there is always at least one index case.

Noninfectious diseases, including genetic conditions and genetic mutations, often also have an index case. In heritable conditions, the index case has a positive family history of the mutation or the condition. An example is a child who is diagnosed with an autoimmune disease that is found to be caused by a genetic mutation and subsequent discovery indicates that the child’s mother carries the same mutation but is asymptomatic for the disease.