Damien’s earlier post  called into question one commonly-held view of the cause of the spread of HIV in Africa, namely male promiscuity.
A paper by Pauline Leclerc and others  (hat tip to Mark Gersovitz) seems to show that there is even greater uncertainty. Leclerc and co-authors tried to simulate the dynamics of the epidemic in Zambia but found that the parameters needed to fit epidemiological models were beyond what the data would allow.
In short, thirty years later, it appears as if we still don’t know what caused the disease to spread the way it did on the continent. Perhaps there is no single set of causes, and that the evolution of the disease is different in different parts of Africa. Perhaps we should move beyond epidemiological models and look to other disciplines for the answers.
At any rate, to fight the epidemic effectively, we need to know how and why it became an epidemic.