What does the term 'index' refer to in outbreak investigations?

• A. First case to be diagnosed
• B. First case in a group to come to the attention of the investigator
• C. The most severe case
• D. The last case reported

Answer: (B)

In outbreak investigations, the index case is the first case in a group that comes to the attention of investigators. This person triggers the investigation and helps set the starting point for the timeline and data collection, such as the epidemic curve and exposure assessment. It’s important to note that the index case is not necessarily the first person who was infected (the primary case) and not necessarily the most severe; someone else may have been infected earlier or more severely ill but not yet identified. In some outbreaks, several cases are detected around the same time, and the one who first reports illness to health authorities becomes the index case, guiding the initial case-finding and interview efforts.