  A two-sex Basic Reproduction Number (BRN) is used to investigate the
conditions under which the Human Immunodeficiency Virus (HIV) may spread
through heterosexual contacts in Sub-Saharan Africa. (The BRN is the expected
number of new infections generated by one infected individual; the disease
spreads if the BRN is larger than 1). A simple analytical expression for the
BRN is derived on the basis of recent data on survival rates, transmission
probabilities, and levels of sexual activity. Baseline results show that in the
population at large (characterized by equal numbers of men and women) the BRN
is larger than 1 if every year each person has 82 sexual contacts with
different partners. the BRN is also larger than 1 for commercial sex workers
(CSWs) and their clients (two populations of different sizes) if each CSW has
about 256 clients per year and each client visits one CSW every two weeks. A
sensitivity analysis explores the effect on the BRN of a doubling (or a
halving) of the transmission probabilities. Implications and extensions are
discussed.
