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Index case

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The index case or patient zero is the initial patient in the population sample of an epidemiological investigation. Patient zero is a somewhat less specific term than index case and is sometimes used to refer to the central patient in an epidemiological investigation rather than the first patient. When used in general to refer to such patients in epidemiological investigations, the term is not capitalized. When the term is used to refer to a specific person in place of that person's name within a report on a specific investigation, the term is capitalized as Patient Zero. Often scientists search for the index case to determine how the disease spread and what reservoir holds the disease in between outbreaks. Note that the index case is the first patient that indicates the existence of an outbreak. Earlier cases may be found and are labelled primary, secondary, tertiary, etc.

The origin of the term "Patient Zero"

In the early years of the AIDS epidemic, there was a lot of controversy about a so-called Patient Zero who was the basis of a complex transmission scenario compiled by Dr. William Darrow and colleagues at the Centers for Disease Control (CDC) in the US. This epidemiological study showed how 'Patient O' (for "Out of California" and mistakenly identified in the press as 'Patient Zero') had given HIV to multiple partners, who then in turn transmitted it to others and rapidly spread the virus to locations all over the world (Auerbach et al., 1984). In all, at least 40 of the 248 people diagnosed with AIDS by April 1982 were thought to have had sex either with him or with someone who had.


  • Auerbach DM, Darrow WW, Jaffe HW, Curran JW. (1984) Cluster of cases of the acquired immune deficiency syndrome. Patients linked by sexual contact. Am J Med. 76, 487-492 PMID 6608269

External links

it:Paziente zero
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