This is a working version of the epidemic model. It creates 400 healthy (blue) turtles and 4 sick (red) turtles and has them move around Spaceland randomly (with a wiggle procedure). The turtles only turn red on collision if the collidee is red.
To continue the epidemic model, REMIX this model, then:
Create a slider to control the initial population of healthy turtles (with a range of 0-500)
Add a graph to count the blue and red turtles
Extensions: Create a slider to control the transmission rate (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.
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.