Index Case: Definition and Role in Outbreak Investigation

The index case is the first identified patient in a disease outbreak. This person may be the source of infection or may have acquired it from an unknown source.

Importance

Identifying the index case is critical for public health officials to:

• Trace transmission chains.
• Identify exposed contacts.
• Implement control measures.
• Understand the outbreak's origin and spread.

The index case is not necessarily the "patient zero" (the very first infected individual), but the first one recognized by the health system.