That’s a mouthful, but the concept is really pretty straightforward. In developing nations with migratory populations, there are parts of the year when everyone tends to meet up, creating localized, seasonal population booms. Those booms, with a large number of people in a concentrated area, are a hotspot for the spread of disease. Unfortunately for health agencies, tracking migratory populations has never been an easy task, which means epidemic can get out of hand quickly."
Link to Original Source