The last numbers I heard for HIV infected needlesticks on healthcare workers conversion to HIV positive is about 1 in 200. Most HIV positive persons are immunocompromised already or recieve massive innoculations (blood or blood product transfusions). Gay men get tears in their rectums with stool mingling with their blood and hence immuno-compromise. IV drug users use dirty needles and the same happens. Hookers have multiple parters and often use IV drugs to boot. In Africa the people just have poor diets and general poor health. When any of these groups get exposed they readily convert.
I'm sure some suvive a long time without the meds 'cause they change their habits and eat well and exercise. However, habits and lifestyles are hard to change and Gay men are often very very promiscuous; not politically correct but fact; thus increasing their exposure. Coupled with the an already weakened immune system it is no mystery why their HIV rate is so high.