The most modern, professional health facility on this side of Lake Atitlan is Hospitalito in Santiago, which serves the local population of 50,000 Maya and a growing ex-pat community.
Savvy locals from San Pedro and San Marcos also go here for lab work, X-rays, treatment and a full range of other medical services. Funded by donations, Hospitalito needs to buy a new ambulance to better serve the community. For more information, email the treasurer (David at Posada Santiago) at Help Hospitalito Santiago.
This link tells more about Hospitalito Santiago.