Adeadly, very contagious disease, carried by fleas and rats migration stream 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 diffusion 4. a group of people who move from one area to another area during the same time period bubonic plague 5. a worldwide spread of a deadly disease, e. g., the plague pandemic