Let's say you'd like to go for a run in Mexico City.
Dr. Tonatiuh Barrientos, an epidemiologist with Mexico's National Institute of Public Health, thinks that's a good idea — in theory. An expert on diabetes, he'd like to see more people in the Mexican capital get out and exercise to combat the disease.
But as a runner himself, he knows that Mexico City isn't an easy place to jog. In a metropolis of 22 million, there are only a handful of parks where people can run.