The index case or primary case 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. "Patient Zero" was used to refer to the index case in the spread of HIV in North America.
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, there was controversy about a so-called “patient zero”[why?], who was the basis of a complex transmission scenario 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 the first person to bring HIV from Africa to the United States and to introduce it to 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 USA 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, a Robert Rayford died in St. Louis, Missouri, of complications from AIDS in 1969, and most likely became infected in the 1950s, 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. 
Other index patients
- Mary Mallon (a.k.a. “Typhoid Mary”) was an index case of a typhoid outbreak. An apparently healthy carrier, she infected 47 people while working as a cook. She eventually was quarantined 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 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 may be erected in his honor. Maria Adela Gutierrez, who contracted the virus about the same time as Hernández, became the first officially confirmed fatality.
In the media, in fiction
In reports and documentaries
In season 10, Radiolab did an hour-long segment on “patients zero”.
- 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.
- At Dead Rising 3 The Protagonist Nick Ramos is "Patient Zero" of zombification, And he likely cured from zombification.
- "Diseases - Activity 1 - Glossary, page 3 of 5 ". science.education.nih.gov. Retrieved 2010-11-03.
- "WordNet Search - 3.0". Princeton University, wordnetweb.princeton.edu. Retrieved 3 November 2010.
- "Sporadic STEC O157 Infection: Secondary Household Transmission in Wales". Centers for Disease Control and Prevention, USA, www.cdc.gov. 1 January 1994. Retrieved 3 November 2010.
- "Patient Zero - definition of Patient Zero in the Medical dictionary - by the Free Online Medical Dictionary, Thesaurus and Encyclopedia". medical-dictionary.thefreedictionary.com. Retrieved 3 November 2010.
- "Definition of index case". The free medical dictionary by farlex.
- "The Origin of HIV and the First Cases of AIDS". AVERT, www.avert.org. Retrieved 2010-11-03.
- Pence, G. E. (2008). Preventing the Global Spread of AIDS. In Medical Ethics Accounts of the Cases That Shaped and Define Medical Ethics (p. 331). New York, USA, McGraw-Hill.
- Matt & Andrej Koymasky - Famous GLTB - Gaëtan Dugas[dead link]
- Bowdler, Neil (2007-10-30). "Key HIV strain 'came from Haiti'". BBC News. Retrieved 2010-05-05.
- "Have Doctors Found Swine "Patient Zero?"". CBS News. 2009-04-29.
- "Researchers trawl for Conficker's 'Patient Zero' - Techworld.com". news.techworld.com. Retrieved 2010-11-03.
- "Patient Zero". TV.com. 2006-03-20. Retrieved 2010-11-03.
- Lemos, Robert. "Witty worm traced to 'Patient Zero'". The Register.
- "That Man in the White House". The Weekly Standard. 28 November 2003. Retrieved 3 November 2010.
- "NOVA | The Most Dangerous Woman in America | In Her Own Words". PBS. 1938-11-11. Retrieved 2010-11-03.
- [dead link]
- "How SARS changed the world in less than six months". Bulletin of the World Health Organization 81 (8). 2003.
- Laurance, Jeremy (2003-04-24). "One family went on holiday – and made Toronto a global pariah". The Independent (London). Retrieved 2010-05-05.
- "Have Doctors Found Swine "Patient Zero?"". CBS News. 2009-04-29. Retrieved 2010-11-03.
- Politician’s Novel Idea for Mexican Tourism: Statue of Swine Flu Survivor
- 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. doi:10.1016/0002-9343(84)90668-5. PMID 6608269.
- "Mapping the Spread of Viruses / Contagions via Contact Tracing". Orgnet, www.orgnet.com. Retrieved 2010-11-03.
- Letter to the New York Review of Books (vol. 35, number 19, December 8, 1988) by Dr. Andrew Moss, Dept. of Epidemiology and International Health, San Francisco, regarding the “patient zero” myth.
- Radiolab Episode "Patient Zero".