The estimation method can be illustrated by employing a simple renewal process which adheres to the original definition of R 0 [1] . Let j(t) be the number of new infections (i.e., incidence) at calendar time t. Supposing that each infected individual on average generates secondary cases at a rate A() at time  since infection (where  is referred to as the "infection-age" hereafter), j(t) is written as: