You might need to be a little more specific. In the US, you can work with the Indian Health Service as part of the US Public Health Service. You can also go overseas after finishing residency to work with MSF (Les Medecin Sans Frontieres - Doctors Without Borders). Likewise, there are several to many religious groups for whom you can go on medical missions overseas - the Mormons and Seventh-Day Adventists are two that I can think of, but there are many others - and, you don't have to proselytize - you may go under their auspices, but you practice medicine. Still, you have to look separately with each organization for specifics. Try searching online for "medical missions".
