They should be responsible since they are in a position to do something about it. They have the money and the resources and should act in the benefit of humanity to ensure that this disease is taken care of. Plus they owe it to their population who will continue to become more and more infected as the disease is spread.
Yes, developed countries are the only countries that have the necessary resources required for the research. Developed countries also have the trained scientific communities that are lacking in under developed countries. AIDS continues to be a growing concern in under developed countries, so it remains up to developed nations to collaborate and find the cure.
All countries should help find a cure for AIDs, and especially developed nations that have the ability to do so. AIDs is a very serious disease that poses a risk to people everywhere. It is already all but out of control in some lesser developed nations, and there should be a lot of effort put into finding a cure.
AIDS affects different areas at different rates. While Africa has a huge problem that doesn't indicate that AIDS is not affecting the world, because it is. I believe developed countries should be responsible for finding an AIDS cure and when it is available it should be provided to as many people as possible. My understanding is that we are coming close, but we can not get rid of this problem by only treating those in developed countries.