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.