By decreasing the amount of wealth that is necessary for a human person to remain alive - for example by introducing a new vaccine that allows a large (5+) family in unsanitary conditions to have all of their children reach adulthood - these sorts of programs in undeveloped countries actually create more poverty then they solve. There is not a cheap-manual labor shortage in these countries. There is however an acute shortage of well-paying jobs that don't involve corruption. By allowing the supply of just-surviving people to increase, you are putting even more downward pressure on the wages of unskilled labor (they are caught in the Iron Law of wages).
Keep in mind that in these countries, with no real legal system, unskilled labor doesn't mean gardening like it does in California but violence in the form of gangs/thug-ocracies who typically control access to natural resources or drug running routes. By creating more people, you are making it easier for the systems that keep them poor to grow, in a positive feedback loop.
The problem is not that a $3 drug is too expensive, and that we need to develop a 30 cent drug, the problem is that there are people who can't afford $3 for a life-saving drug. What they really need from the US is jobs, markets, and protection from the threats of climate change that we are causing by using 25% of the world's fossil fuels (more if you count the contribution from production in China to the cheap goods that we consume).
If Gates really wanted to help these people, he would campaign for an end to developed world farm subsidies, fossil fuel subsidies, and the recreational diamond market. But that would make him unpopular in the US.
EDIT: are downvotes for disagreement or for breach of etiquette?