Discussion of existing hypothesis about novel factors driving HIV origin and epidemic emergence.

What is it about?

We present evidence that novel factors (factors present in tropical Africa only from late 19th century onward) were necessary for HIV to emerge epidemically in humans. This can be demonstrated from the high prevalence of unproductive human infections by SIV, the chronic human exposure to SIV through bushmeat handling, and the lack of HIV emergence before late 19th century. We review the Parenteral Serial Transmission and the Enhanced Heterosexual Transmission hypotheses for the adaptation and early spread of HIV. Epidemic start roughly coincides in time with peak genital ulcer disease in cities, suggesting a major role for sexual transmission. Only ill-adapted and rare HIV groups emerged after approximately 1950, when injections and transfusions attained their maximal levels, suggesting that if parenteral serial transmission was necessary for HIV adaptation, it had to be complemented by sexual transmission for HIV to reach epidemic potential.

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Why is it important?

Understanding the factors that permitted HIV groups to emerge may be important to prevent new SIVs to take hold in the human population, and it may provide a template to understand the zoonotic origins of other major human infectious diseases. A high spatial and temporal concentration of co-factors of transmission may, for example, be important also to evaluate whether Ebola outbreaks could spark a pandemic. Some outstanding questions and prospects for future research are summarized in section Future perspective.

Perspectives