Critical Thinking Activity with FRED Models

How does herd immunity affect the spread of an infectious disease?

Here is an excellent demonstration of the effect of herd immunity on the spread of an infectious disease. Herd immunity is the protection of the unvaccinated portion of a population because there is smaller chance of unvaccinated people coming in contact with an infected person. Thus, an infectious disease cannot readily spread.

This link provides two FRED simulations of the outbreak of measles in the state and town of your choosing:

Once you select the town, two maps will appear of the county containing that town. Click the black arrow at the bottom left of each map and watch the number of infected people rise and then fall as days pass. Eighty percent of the community depicted in the model results shown in the left map were vaccinated against measles, and 95% were vaccinated in the right map. Watch the spread of measles in these two maps and fill in the tables below:

Location: County: State:
Vaccination Rate Days to Max Infections Days to Outbreak


  1. Was the measles outbreak large, medium or small with 80% vaccination rate?
  2. Was the measles outbreak large, medium or small with 95% vaccination rate?
  3. What would you guess is the vaccination rate needed to insure herd immunity for measles?
  4. What clues do the maps provide about the geographical controls of disease spread?
  5. Are you vaccinated for measles?