These kinds of questions get contradictory answers because everyone's experience is different, so it doesn't necessarily generalize to other people. The basic mindset that you need is to remember that bacteria multiply far more quickly at ambient temperatures in the tropics than in temperate zones (anyone doubting this is welcome to take a casual approach to other bacterial issues, like minor scratches and wounds, which might be benign in the USA or Europe, but can get serious very quickly in the tropics). To complicate matters, water sources, surfaces and the busy little feet of common flies (which in some places run rampant) are often far more polluted than at home; sewage disposal is often, umm, unsophisticated, and you know where flies like to walk around. Finally, you may not have developed resistance to the local microbes; to some degree, you probably already have at home.
My experience with many Africans is that they wash more frequently than lots of tourists. What's more, most of Africa is nowhere near as bad for G.I. tract ills as is, say, south Asia. You can often get away with eating street food or drinking form the local water supply in ways which would cripple you in Nepal or India. But that doesn't make it a good idea, especially when you've got a choice.
In all of the above, we're ignoring the related issue of worms. I've known people to get worms in Africa, although I never have myself, and it doesn't look like a pleasant experience. Fortunately, the same precautions which apply to bacterial infections also prevent the sorts of worms found in foods.
And don't forget that some bacteria produce toxins which survive cooking; that means that food which has been left out for too long before cooking might poison you even though the bacteria themselves have been killed by the heat of cooking. Just in case you need something more to consider at your leisure.
This message brought to you by your local nagging worrywart.
Mark
(who generally eats whatever he wishes (within reason) in Africa, but carries a fairly complete medical kit and doesn't hesitate to use it)