In this paper we examine the spread of HIV when this disease is transmitted through the random sharing of contaminated drug injection equipment. We first model the spread of disease using a standard set of behavioral assumptions discussed by Kaplan . We demonstrate that deterministic and stochastic models based on these assumptions behave very similarly and use a branching process approximation to show that if the basic reproductive number, R0, is less than or equal to unity then the disease will always become extinct. If R0>1 then, although the disease might take off, it is still possible for it to die out, and we calculate the probability of extinction. This is not of the simple form R0-a, where a is the initial number of infectious addicts, which might have been expected from Whittle's stochastic threshold theorem . We next discuss an extended model that incorporates a three-stage AIDS incubation period and again examine a branching process approximation. We finally explore the extent to which control strategies such as needle exchange and improved needle cleaning can reduce the risk of a HIV epidemic before concluding with a brief discussion.
- stochastic models