What would actually happen if only 80 percent of school-age children were vaccinated against the measles? It’s a scary thought that a new simulation from the University of Pittsburgh aims to visualize.

On the Framework for Reconstructing Epidemiological Dynamics, or FRED for short, users can select a state and a city to see how two very different scenarios would play out over 238 days. The first example from FRED, below, shows what would happen in Manhattan if 80 percent of school-age children were vaccinated. The second example shows the result if the borough had 95 percent vaccination coverage.

READ MOREThis article originally appeared on Scientific American‘s website on March 9, 2015.