When I was a kid I did Unicef collection every Haloween. We got an orange cardboard coin box at school, and collected donations to it along with our trick-or-treat. Unicef used these funds to build water wells for people in Africa who had only access to contaminated surface water.
A decade or two later, we found that many of these wells accessed aquifers that were contaminated by arsenic. And that thus we kids had funded the wholesale poisoning of people in Africa, and that a lot of them had arsenic-induced cancers that were killing them.
OK, we would not make that mistake again, and today we have access to better water testing. But it caused me to lose my faith that we really do know how to help poor people in the third world, no matter how well-intentioned we are.
And we had better not go around curing disease withoput also promoting birth control. Despite what the churches say, and the local dislikes and prejudices. Or we'll just be condemning more people to starve.