The index case, primary case, or patient zero is the initial patient in the population of an epidemiological investigation, or more generally, 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 will sometimes achieve the status of a "classic" case in the literature, as did Phineas Gage.
The index case may indicate the source of the disease, the possible spread, and which reservoir holds the disease in between outbreaks. The index case is the first patient that indicates the existence of an outbreak. Earlier cases may be found and are labeled primary, 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 genetics, the index case is the case of the original patient (propositus or proband) that stimulates investigation of other members of the family to discover a possible genetic factor.
Gaëtan Dugas case ("Patient Zero")
In the early years of the AIDS epidemic, a “patient zero” transmission scenario was compiled by Dr. William Darrow and colleagues at the U.S. Centers for Disease Control and Prevention (CDC). This epidemiological study showed how “patient zero” had infected multiple partners with HIV, and they, in turn, transmitted it to others and rapidly spread the virus to locations all over the world (Auerbach et al., 1984). The CDC identified Gaëtan Dugas as a carrier of the virus from Europe to the United States and spreading it to other men he encountered at gay bathhouses.
Journalist Randy Shilts subsequently wrote about Patient Zero, based on Darrow's findings, in his 1987 book And the Band Played On, which identified Patient Zero as Gaëtan Dugas. Dugas was a flight attendant who was sexually promiscuous in several North American cities, according to Shilts' book. He was vilified for several years as a "mass spreader" of HIV, and seen as the original source of the HIV epidemic among homosexual men. Four years later, Darrow repudiated the study's methodology and how Shilts had represented its conclusions.
A 2007 study published in the Proceedings of the National Academy of Sciences of the United States by Michael Worobey and Dr. Arthur Pitchenik claimed that, based on the results of genetic analysis, current North American strains of HIV probably moved from Africa to Haiti and then entered the United States around 1969, probably through a single immigrant. However, Robert Rayford died in St. Louis, Missouri, of complications from AIDS in 1969, and most likely became infected before 1966, so there were prior carriers of HIV strains in North America.
The phrase "patient zero" is now used in the media to refer to the index case for infectious disease outbreaks, as well as for computer virus outbreaks, and, more broadly, as the source of ideas or actions that have far-reaching consequences.
David Heymann, infectious-disease epidemiologist at the London School of Hygiene & Tropical Medicine, has questioned the importance of finding patient zero and has said: “Finding patient zero may be important in some instances, but only if they are still alive and spreading the disease. And more often than not, especially in large disease outbreaks, they’re not."
Other index patients
- Mary Mallon (a.k.a. “Typhoid Mary”) was an index case of a typhoid outbreak in the early 1900's. An apparently healthy carrier, she infected 47 people while working as a cook. She eventually was isolated to prevent her from spreading the disease to others.
- The first recorded victim of the Ebola virus was a 44-year-old schoolteacher named Mabalo Lokela, who died on 8 September 1976, 14 days after symptom onset.
- 64-year-old Liu Jianlun, a Guangdong doctor, transmitted SARS during a stay in the Hong Kong Metropole Hotel in 2003.
- A baby in the Lewis House at 40 Broad Street is considered the index patient in the 1854 cholera outbreak in the Soho neighborhood of London. (The Ghost Map, Steven Johnson, 2005.)
- Édgar Enrique Hernández may be patient zero of the 2009 swine flu outbreak. He recovered, and a bronze statue has been erected in his honor. Maria Adela Gutierrez, who contracted the virus about the same time as Hernández, became the first officially confirmed fatality.
- Two-year-old Emile Ouamouno is believed to be the index patient in the 2014 Ebola epidemic in Guinea and West Africa.
In the media
In journalism and documentaries
- The film Outbreak focuses around the search for a patient zero of an epidemic.
- The novel Rant stars the character Rant Casey, patient zero of the book's rabies epidemic.
- The phone game Plague Inc. can have the CDC look for Patient Zero for more info on the player's virus.
- In the video game Dead Rising 3 the protagonist, Nick Ramos, is patient zero of the zombification phenomenon.
- The film Contagion character Elizabeth Emhoff is patient zero of the fatal MEV-1 virus.
- In the Planet of the Apes film series, Robert Franklin (portrayed by Tyler Labine) is Patient Zero of the Simian Flu Pandemic.
- In the game Prototype, Alex Mercer steals something in GenTek labs and becomes the unwilling patient zero for the virus the lab created.
- In the TV series Fear The Walking Dead, Gloria is speculated to be patient zero in the show's pilot.
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- Patient-Zero Not to Blame The Scientist 
- Worobey, Michael et al "1970s and ‘Patient 0’ HIV-1 genomes illuminate early HIV/AIDS history in North America" Nature (2016) doi:10.1038/nature19827
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- Radiolab Episode "Patient Zero".
- Index Case Lyrics