The problem with giving billions to charity is that it is treating symptoms not causes. Lets say that the results of disease in africa are related to the on-going warfare and lack of developement in the area. Okay, what (mainly) led to this warfare? Western intervention. Why does the West get involved? Because africa sits on some of the largest reserves of raw materials on earth (see the Congo and Coltan which is pretty much feeding our lovely electronics industry).
http://en.wikipedia.org/wiki/Coltan
Why does the west have such an appetite for these resources? Because it constantly requires growth and expansion.
Bill Gates (and Warren Buffet et al) from one hand operate the machinery of international capitalism and give us all it's horrific results, then from the other give us part of the money they made exploiting the world to fix some of the symptoms of their own actions.
You might argue that money would be better spent trying to dismantle the current political-economy and replace it with one that doesn't require most of the world to live in poverty.