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

R0 is a measure of pathogen virulence quantified solely by case-fatality ratio and does not relate to transmission dynamics.

R0 is the time between exposure and symptom onset for the median case in an outbreak and has units of days.

R0 is the average number of secondary cases produced by a single infectious individual in a completely susceptible population, absent any interventions.

R0 is the proportion of a population that must be vaccinated to achieve herd immunity for any given disease, regardless of its transmission characteristics.