what about the possibility of a variable "success" rate of infection
based on how long the virus has been resident in the body, for example,
a persons immune system actively fighting the virus will show a low
amount of the virus in the blood and therefore exhibit less infectivity,
also a person succumbing to the virus with its numbers rocketting in the
blood may well have a very high infectivity with a significant chance of
i am not sure the assumption of infection as a constant can't be
bettered by a slightly more dynamic program which will reflect the
states of people with the disease. also what about the behavioural
aspects of those people who know/do not know that they have the disease.
Its just a thought anyway...