The index case is the first documented case of a disease and is important in studying epidemics to understand the origins, spread, and potential natural reservoirs of the disease. Epidemiologists create “family trees” to trace the spread of the disease. The index case may be the starting point of a new or previously known disease outbreak. Identifying the index case early allows for studying the conditions and structure of the disease, as well as the living conditions, wildlife, and customs of the region. The index case can provide important clues for tracing the course of the disease and identifying potential causes.
In epidemiology, an index case is the first documented case of a disease. When studying epidemics, the identification of these cases can provide very important clues about the origins of the epidemic, how it spread, and whether or not there is a natural reservoir of the disease that could cause future epidemics. As a result, epidemiologists build complex “family trees” to examine the ways in which epidemics spread, while looking for the index case.
Some people refer to the index case as “patient zero,” and epidemiologists will sometimes use this slang term, especially when they wish to protect the identity of the case. In some 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 was simply in the wrong place at the wrong time.
An index case may represent an outbreak of an entirely new disease, as in the case of Mabalo Lokela, who famously was the first known person to die of Ebola, or may be the first person involved in an outbreak of a previously known disease; Typhoid Mary is a famous example of such a case. In both cases, these people were the starting point of the epidemic, and once the epidemic was traced back to these individuals, people could learn a lot about how the epidemic developed and spread.
Ideally, epidemiologists like to identify the index case as early as possible. Once identified, scientists can travel to the region where it lives to study its conditions, in hopes of learning more about how the epidemic developed. While on site, the epidemiologists will take samples from the index patient so they can study the structure of the disease, and will also examine living conditions, local wildlife and regional customs to fit the pieces of the puzzle together. If possible, the index patient is interviewed so that researchers can monitor his movements and activities while he is infected with the disease.
When an index case is identified, it is common to have large chunks of the outbreak family tree on hand. These pieces can be used to chart the course of the disease and learn more about the next steps that should be taken. For example, in outbreaks caused by contaminated food, by linking known facts about the index case and other victims, epidemiologists may be able to figure out what they had in common, allowing them to focus on one cause, such as a mishandled batch of meat or contaminated products.
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