Consider volunteering at one of the Indian Health Service clinics/hospitals, or trying to find mission trips that might go to one of the reservatiosn. Per Wikiepdia about the Pine Ridge Indian Reservation (comprising some of the poorest counties in the US):
- As of 2011, population estimates of the reservation range from 28,000 to 40,000. Numerous enrolled members of the tribe live off the reservation.[61]
- 80% of residents are unemployed;
- 49% of the residents live below the Federal poverty level (including 61% of all those under the age of 18);
- Per capita income in Oglala Lakota County is $6,286;
- The infant mortality rate is 5 times higher than the national average;
- Native American amputation rates due to diabetes is 3 to 4 times higher than the national average;
- Death rate due to diabetes is 3 times higher than the national average; and
- Life expectancy in 2007 was estimated to be 48 for males and 52 for females
My friends set up an Appalachian service mission while in medical school and did a world of good. If you're in medical school you can likely set one up too.
Unless you speak the language of the foreign country you're considering, you can likely do far more good staying in the US. The main thing medical students and pre-meds can do to help others is education. That's incredibly difficult when you're reliant on an interpreter for communication. You will learn a ton about other cultures by visiting them, and that in of itself is quite valuable, but I'd argue you can still do do that in Appalachia or at in Indian reservation, and make a bigger impact.
FYI I say this being a huge hypocrite myself--I never did a service trip (I volunteered my summers away at the local free clinic for dayworkers as a pre-med--I just enjoyed that more), and in medical school I tried to plan the service trip with my friends to Appalachia (my vote was for an Indian Reservation, but logistics favored Appalachia), but I unfortunately couldn't make it (logistics also favored scheduling it when my now-wife and I had planned our wedding).
So take my advice with a grain of salt--I'm not trying to criticize people that do foreign service trips, rather just trying to point out some very needy and often overlooked groups within the US.