If internal and international migrants comprised a nation, it would be the third most populous country in the world, just after China and India. Thus, there can be little doubt that population mobility is among the leading policy issues of the 21st century. However, policies to protect migrants and global health have so far been hampered（阻碍，限制） by inadequate policy attention and poor international coordination. This is the conclusion of a new article in PLoS Medicine arguing that current policy-making on migration and health has been conducted within sector silos（粮仓） , which frequently have different goals. Yet, population mobility is wholly compatible with health-promoting strategies for migrants if decision-makers coordinate across borders and policy sectors, say the authors, who are also serving as guest editors of a new series in PLoS Medicine on migration & health that launches this week. In the first article in the series, Cathy Zimmerman, Ligia Kiss, and Mazeda Hossain from the London School of Hygiene & Tropical Medicine (LSHTM), United Kingdom, lay out a new framework that describes migration as a multi-staged process involving numerous health risks and intervention opportunities. This migration-health framework benefits international and local policy-makers by highlighting various points for intervention and options for coordination between nations. Five subsequent articles in the series discuss in-depth the health impacts and policy needs associated with the five phases of this migratory process: pre-departure, travel, destination, interception, and return.

The second article in the series, by Brian Gushulak and Douglas MacPherson from Migration Health Consultants in Canada, is also published in this week's PLoS Medicine and discusses the specific health risks and policy needs associated with pre-departure. Subsequent articles in the series, to be published weekly until 21 June 2011, will discuss the remaining stages of the migration process.