1. a deadly, very contagious disease, carried by fleas and rats diffusion 2. a group of people who have recently moved to a new area and who return to their original location during the same time period countermigration stream 3. the movement of a characteristic from one place to another over a period of time pandemic 4. a group of people who move from one area to another area during the same time period migration stream 5. a worldwide spread of a deadly disease, e. g., the plague bubonic plague