The basic reproductive number, R 0 , is the average number of secondary infections produced by an infected individual in an otherwise fully susceptible population [20] . R 0 is the most important parameter in infectious disease epidemiology, and considerable effort has been devoted to its estimation and to understanding its implications for disease control [20, [22] [23] [24] [25] [26] , although it is important to note that some factors not incorporated in R 0 , e.g., host births as well as deaths, may also have important control implications.