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Concept
Index case
Summary
The index case or patient zero is the first documented patient in a disease epidemic within a population, or the first documented patient included in an epidemiological study.
It can also refer to the first case of a condition or syndrome (not necessarily contagious) to be described in the medical literature, whether or not the patient is thought to be the first person affected. An index case can achieve the status of a "classic" case study in the literature, as did Phineas Gage, the first known person to exhibit a definitive personality change as a result of a brain injury.
The index case may or may not indicate the source of the disease, the possible spread, or which reservoir holds the disease in between outbreaks, but may bring awareness of an emerging outbreak. Earlier cases may or may not be found and are labeled primary or coprimary, secondary, tertiary, etc. The term primary case can only apply to infectious diseases that spread from human to human, and refers to the person who first brings a disease into a group of people. In epidemiology, the term is often used by both scientists and journalists alike to refer to the individual known or believed to have been the first infected or source of the resulting outbreak in a population as the index case, but such would technically refer to the primary case.
"Patient zero" was used to refer to the supposed source of HIV outbreak in the United States, flight attendant Gaëtan Dugas in the popular press, but the term's use was based on a misunderstanding (and Dugas was not the index case). In the 1984 study of Centers for Disease Control and Prevention (CDC), one of the earliest recorded HIV-patients was code-named "patient O", which stands for "patient out of California". The letter O, however, was interpreted by some readers of the report as the numeral 0. The designation patient zero (for Gaëtan Dugas) was subsequently propagated by the San Francisco Chronicle journalist Randy Shilts in his book And the Band Played On in 1987.
Official source
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