The first case of a contagious disease in a group or population that serves to call attention of community health personnel for presence of the disease:

Epidemiological terminology The index case is the first case of a contagious disease in a group or population that serves to call the attention of community health personnel for presence of the disease.  It may not always be the primary case. Index case is the case of the original patient (prepositus or proband) that stimulates investigation of other members of the family to discover a possible genetic factor (in case of genetic research). Primary case is the first case of communicable disease introduced into the population unit under study. Person who acquires the disease from an exposure. Secondary cases are those arising contact with the primary case. Person who acquires the disease from an exposure to the primary case.