What is the definition of the basic reproduction number (R0) in infectious disease epidemiology?

• A. R0 is a measure of pathogen virulence quantified solely by case-fatality ratio and does not relate to transmission dynamics.
• B. R0 is the time between exposure and symptom onset for the median case in an outbreak and has units of days.
• C. R0 is the average number of secondary cases produced by a single infectious individual in a completely susceptible population, absent any interventions.
• D. R0 is the proportion of a population that must be vaccinated to achieve herd immunity for any given disease, regardless of its transmission characteristics.

Answer: (C)