This is a working version of the colliding turtles model, which students can use as the basis for an epidemic model. It creates 300 healthy (blue) turtles and 5 sick (red) turtles and has them move around Spaceland randomly (with a wiggle). The turtles only turn red on collision if the collidee is red.
To make an epidemic model, REMIX this model, then:
Create a transmission rate slider with a range from 0 to 100. Use the transmission slider in the code as the probability of passing on the disease when a turtle collides with a sick turtle.
Next, create a recovery slider with a range from 0 to 10, and use the recovery slider in the code as the probability that a turtle will recover from the disease at each step (and so become healthy, or blue). Since each turtle has a chance of recovering at each step, this procedure should be called in a forever block, not in the collision block.