in my opinion, Infectious Disease is definitely one of the best ways to go if you are truly devoted to international work. ID docs not only have ID, but also general medicine in their background.
Gen surg, Internal Medicine, Peds, Plastics, Gyn, and even Pathologists are helpful for international work. That is one of the reasons I chose to do Internal Medicine over PM&R (what I initially started out with). Its good to have the basics of medicine or surgery to do this kind of work. i just didn't see myself being as helpful if i had become a physiatrist (although i'm sure its still possible to do that work but you wouldn't just have as much medicine experience).
there are so many places that need our help.
also, what is the general opinion on the "learn as you go" idea as proposed by the last poster?!?