immune(t) = immune(t - dt) + (getting_better) * dt

INIT immune = 0



INFLOWS:

getting_better = infected*recovery_rate

infected(t) = infected(t - dt) + (getting_sick - getting_better) * dt

INIT infected = 1



INFLOWS:

getting_sick = infected*susceptible*infection_rate
OUTFLOWS:
getting_better = infected*recovery_rate
susceptible(t) = susceptible(t - dt) + (- getting_sick) * dt
INIT susceptible = 1000
OUTFLOWS:
getting_sick = infected*susceptible*infection_rate
infection_rate = 1/500
recovery_rate = 1/2