Example base simulation

turtles-own
  [ sick?                ;; if true, the turtle is infectious
    remaining-immunity   ;; how many weeks of immunity the turtle has left
    sick-time            ;; how long, in weeks, the turtle has been infectious
    age ]                ;; how many weeks old the turtle is

globals
  [ %infected            ;; what % of the population is infectious
    %immune              ;; what % of the population is immune
    lifespan             ;; the lifespan of a turtle
    chance-reproduce     ;; the probability of a turtle generating an offspring each tick
    carrying-capacity    ;; the number of turtles that can be in the world at one time
    immunity-duration ]  ;; how many weeks immunity lasts

;; The setup is divided into four procedures

to setup
  clear-all
  setup-constants
  setup-turtles
  update-global-variables
  update-display
  reset-ticks
end 

;; We create a variable number of turtles of which 10 are infectious,
;; and distribute them randomly

to setup-turtles
  create-turtles number-people
    [ setxy random-xcor random-ycor
      set age random lifespan
      set sick-time 0
      set remaining-immunity 0
      set size 1.5  ;; easier to see
      get-healthy ]
  ask n-of 10 turtles
    [ get-sick ]
end 

to get-sick ;; turtle procedure
  set sick? true
  set remaining-immunity 0
end 

to get-healthy ;; turtle procedure
  set sick? false
  set remaining-immunity 0
  set sick-time 0
end 

to become-immune ;; turtle procedure
  set sick? false
  set sick-time 0
  set remaining-immunity immunity-duration
end 

;; This sets up basic constants of the model.

to setup-constants
  set lifespan 50 * 52      ;; 50 times 52 weeks = 50 years = 2600 weeks old
  set carrying-capacity 300
  set chance-reproduce 1
  set immunity-duration 52
end 

to go
  ask turtles [
    get-older
    move
    if sick? [ recover-or-die ]
    ifelse sick? [ infect ] [ reproduce ]
  ]
  update-global-variables
  update-display
  tick
end 

to update-global-variables
  if count turtles > 0
    [ set %infected (count turtles with [ sick? ] / count turtles) * 100
      set %immune (count turtles with [ immune? ] / count turtles) * 100 ]
end 

to update-display
  ask turtles
    [ if shape != turtle-shape [ set shape turtle-shape ]
      set color ifelse-value sick? [ red ] [ ifelse-value immune? [ grey ] [ green ] ] ]
end 

;;Turtle counting variables are advanced.

to get-older ;; turtle procedure
  ;; Turtles die of old age once their age exceeds the
  ;; lifespan (set at 50 years in this model).
  set age age + 1
  if age > lifespan [ die ]
  if immune? [ set remaining-immunity remaining-immunity - 1 ]
  if sick? [ set sick-time sick-time + 1 ]
end 

;; Turtles move about at random.

to move ;; turtle procedure
  rt random 100
  lt random 100
  fd 1
end 

;; If a turtle is sick, it infects other turtles on the same patch.
;; Immune turtles don't get sick.

to infect ;; turtle procedure
  ask other turtles-here with [ not sick? and not immune? ]
    [ if random-float 100 < infectiousness
      [ get-sick ] ]
end 

;; Once the turtle has been sick long enough, it
;; either recovers (and becomes immune) or it dies.

to recover-or-die ;; turtle procedure
  if sick-time > duration                        ;; If the turtle has survived past the virus' duration, then
    [ ifelse random-float 100 < chance-recover   ;; either recover or die
      [ become-immune ]
      [ die ] ]
end 

;; If there are less turtles than the carrying-capacity
;; then turtles can reproduce.

to reproduce
  if count turtles < carrying-capacity and random-float 100 < chance-reproduce
    [ hatch 1
      [ set age 1
        lt 45 fd 1
        get-healthy ] ]
end 

to-report immune?
  report remaining-immunity > 0
end 

to startup
  setup-constants ;; so that carrying-capacity can be used as upper bound of number-people slider
end 


; Copyright 1998 Uri Wilensky.
; See Info tab for full copyright and license.