Final answer:
The individual who first introduces an infectious agent to a population is known as the Index case. This individual is instrumental in the study of epidemics as epidemiologists work to identify the causative agents of infectious diseases.
Step-by-step explanation:
The individual who first introduces an infectious agent to a population is called the Index case. Infectious diseases, such as chickenpox, are caused by pathogens, which include bacteria, viruses, fungi, and protozoa. These diseases are communicable and can spread from person to person. The index case is often involved in the study of an epidemic's etiology, as epidemiologists seek to identify the causative agent or pathogen responsible for the outbreak. Understanding the role of the index case is crucial in tracking the propagation of an infectious disease and in implementing measures to control the spread.