In epidemiology, an index case is the first documented case of a disease. When studying epidemics, identifying these cases can provide very important clues about the origins of the outbreak, how it spread, and whether or not there is a natural reservoir of the disease which might cause future epidemics. As a result, epidemiologists construct complex "family trees" to look at the ways in which epidemics spread, as they search for the index case.
Some people refer to the index case as "patient zero," and sometimes epidemiologists will use this slang term, especially when they wish to protect the identity of the case. In some disease outbreaks, the index case has been the subject of intense interest, with people blaming the individual for the outbreak. This characterization is somewhat unfair, and often Patient Zero simply was in the wrong place at the wrong time.
An index case may represent the outbreak of an entirely new disease, as was the case with Mabalo Lokela, who famously was the first known person to die of Ebola, or it may be the first person involved in an epidemic of an already-known disease; Typhoid Mary is a famous example of such a case. In both instances, these people were the starting point of the epidemic, and once the epidemic was traced back to these individuals, people were able to learn a great deal about how the epidemic developed and spread.
Ideally, epidemiologists like to identify the index case as soon as possible. Once it has been identified, scientists can travel to the region where he or she lives to study the conditions there, in the hopes of learning more about how the epidemic developed. While on site, epidemiologists will take samples from the index patient so that they can study the structure of the disease, and they will also look at living conditions, local wildlife, and regional customs to put the pieces of the puzzle together. If possible, the index patient is interviewed so that researchers can track his or her movements and activities while infected with the disease.
By the time an index case is identified, it is common to have large pieces of the family tree of the epidemic in hand. These pieces can be used to trace the pathway of the disease, and to learn more about the next steps which should be taken. For example, in disease outbreaks caused by tainted food, by linking the known facts about the index case and other victims, epidemiologists may be able to figure out what they had in common, allowing them to zero in on a cause, like a batch of poorly handled meat or contaminated produce.