Here, the above mentioned negative aspect of R a is reconsidered by correcting the definition of R a . The disease of interest in the present study is HIV. The frequency of secondary transmissions relative to infection-age  (i.e., the generation time distribution), approximated by a step function, is shown in Figure 1 . As has been known for some time [19] , the frequency of secondary transmissions is very high shortly after infection (for a duration d 1 = 0.24 years), followed by a long asymptomatic period with a low frequency of secondary transmissions (for d 2 = 8.38 years) [20] . Subsequently, the frequency rises sharply again resulting in a substantial number of secondary cases for a duration d 3 = 0.75 years until death or until the infected individual ceases risky sexual contact due to AIDS [20] [21] [22] . Assuming that the contact frequency does not vary as a function of infection-age, g 1 , g 2 and g 3 have been estimated at 1.30, 0.05 and 0.36 per year [20] . Here, g() is the density function of the generation time, with a mean of 3.79 years.