Index case

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.

A 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 a gay Canadian flight attendant named Gaëtan Dugas (February 20, 1953&mdash;March 30, 1984 ). For several years, Dugas was vilified as a "mass spreader" of HIV and the original source of the HIV epidemic among gay men. However, four years after the publication of Shilts's article, Dr. Darrow repudiated his study, admitting that its methods were flawed and claiming that Shilts had misrepresented the study's conclusions.

Other Patients Zero

 * Mary Mallon (a.k.a. Typhoid Mary) was a real Patient Zero. An apparently healthy carrier of typhoid, she infected 47 people while working as a cook.  She eventually had to be quarantined to prevent her from spreading the disease to others.
 * The first recorded case of the Ebola virus was a 44-year-old schoolteacher named Mabalo Lokela, who died 8 September, 1976 after 14 days of symptom onset.

Patients Zero in Fiction

 * Patient Zero was a character in the Canadian film Zero Patience.
 * In the third Sliders episode, "Fever", Quinn's double is alleged to be Patient Zero of a global plague.
 * The movie Outbreak dealt with government officials charged with finding the index case of an outbreak of an Ebola-like virus.
 * Patient Zero was the name of a minor enemy in the computer game City of Heroes.
 * In Max Brooks's World War Z, a Chinese youth is patient zero for a worldwide zombie epidemic.
 * The 2006 television series Heroes features a super-powered man named Sylar who is described as "Patient Zero" of a genetic experiment into human evolution.
 * In the television show Numb3rs, Patient Zero was a concept used to find the origin of the spread of the Spanish Flu virus.