Index case

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.

"Patient Zero" was used to refer to the supposed 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.

The term can also be used in non-medical fields to describe the first individual affected by something negative that since propogated to others, such as the first user on a network infected by malware.

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.

...
Wikipedia