Database Reference
In-Depth Information
1: INFECTED
1:
60000
1
1:
30000
1
1
0
1:
0
300
600
900
1200
W eeks
Fig. 2.7
To capture the spread of disease in the second population, duplicate the model
of Figure 2.6 by first clicking outside its boundaries and dragging the Hand over
the entire model diagram. Then select Copy from the Edit pull-down menu. Then
click well below the existing model diagram and select Paste from the Edit pull-
down menu. Here is what you should get—two virtually identical versions of the
model, with the names of the second model automatically changed to avoid con-
fusion (Figure 2.8). Change the value of SURVIVAL RATE 2 to 0.20 and that of
CONTACT RATE 2 to 0.003 to reflect the assumptions that the rate of contact
among individuals of this second species are higher but the death rate lower than
for the first species. This is the case, for example for some strains of the AIDS and
Ebola viruses, which seem, at least until these viruses evolve, to have more detri-
mental effects on humans than monkeys.
The two parts of the model in Figure 2.8 are not yet connected with each other.
Create a new contact rate to reflect interaction between the two populations, and
assume that the disease gets passed on from the second to the first population, i.e.
GET SICK
=
CONTACT RATE * NON IMMUNE * INFECTED
+
CONTACT RATE 1 2 * NON IMMUNE * INFECTED 2
(2.7)
Use this contact rate to establish a logical connection between the two parts of the
model. Figure 2.9 shows incorporation of that new contact rate and the correspond-
ing connections between GET SICK and the nonimmune population of the second
species.
If you set CONTACT RATE 1 2 to 0, then the dynamics of the disease in the
first population should be as before (i.e. as in Figure 2.7). For subsequent runs ex-
plore how the results change under different assumptions about the rate of contact
 
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