I'm happy you asked this question. I was about to start a thread concerning Epidemiology, because last year when I was applying I had no idea this was an option for MST programs. I thought basic science was the only way to go. Very fortunately, I happened to interview (and get accepted) to a MSTP school that has (and actually encourages this option!). There are many, many positives for going this route.
At first impression, it appears as though a MD and PhD in Epidemiology may actually work synergistically in a carefully designed career. This is very important point to make, because it seems far too often that the opposite is the case. This is a major selling point to me.
Second, after talking with the Epidemiology faculty at the school I'll be attending next fall, it appears that for summer "lab" rotations there are many options available. Some of what we discussed included a month at the WHO in Geneva! Also, when I discussed with them potential thesis topics, they made it sound as if the thesis topic (as long as the methodology is sound) is completely up to me. So collecting thesis data in a third world country or at this medical school are both valid options.
Also, in this program I'll be attending, the somewhat flexible timeline is arranged as follows: 2yr md, 2yr PhD, 2yr MD (w/ data collection for thesis spread throughout this time period) and 1 yr PhD (or less to write up the thesis).
Please let me know if you have any other questions! Although I haven't started the program yet, I would love to help.