NetLogo Models Library:
IABM Textbook/chapter 6

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Spread of Disease

If you download the NetLogo application, this model is included. You can also Try running it in NetLogo Web

ACKNOWLEDGMENT

This model is from Chapter Six of the book "Introduction to Agent-Based Modeling: Modeling Natural, Social and Engineered Complex Systems with NetLogo", by Uri Wilensky & William Rand.

• Wilensky, U. & Rand, W. (2015). Introduction to Agent-Based Modeling: Modeling Natural, Social and Engineered Complex Systems with NetLogo. Cambridge, MA. MIT Press.

This model is in the IABM Textbook folder of the NetLogo Models Library. The model, as well as any updates to the model, can also be found on the textbook website: http://www.intro-to-abm.com/.

WHAT IS IT?

This model explores the spread of disease in a number of different conditions and environments. In particular, it explores how making assumptions about the interactions of agents can drastically affect the results of the model.

HOW IT WORKS

The SETUP procedure creates a population of agents. Depending on the value of the VARIANT chooser, these agents have different properties. In the NETWORK variant they are linked to each other through a social network. In the MOBILE and ENVIRONMENTAL variants they move around the landscape. At the start of the simulation, NUM-INFECTED of the agents are infected with a disease.

The GO procedure spreads the disease among the agents. In the case of the NETWORK variant this is along the social network links. In the case of the MOBILE or ENVIRONMENTAL variant, the disease is spread to nearby neighbors in the physical space. In the case of the ENVIRONMENTAL variant the disease is also spread via the environment. Finally, if the variant is either the MOBILE or ENVIRONMENTAL variant then the agents move.

HOW TO USE IT

The NUM-PEOPLE slider controls the number of people in the world.

The VARIANT chooser controls how the infection spreads.

NUM-INFECTED controls how many individuals are initially infected with the disease.

The CONNECTIONS-PER-NODE slider controls how many connections to other nodes each node tries to make in the NETWORK variant.

The DISEASE-DECAY slider controls how quickly the disease leaves the current environment.

To use the model, set these parameters and then press SETUP.

Pressing the GO ONCE button spreads the disease for one tick. You can press the GO button to make the simulation run until all agents are infected.

The REDO LAYOUT button runs the layout-step procedure continuously to improve the layout of the network.

THINGS TO NOTICE

How do the different variants affect the spread of the disease?

In particular, look at how the different parameters of the model influence the speed at which the disease spreads through the population. For example, in the "mobile" variant, the population (NUM-PEOPLE) clearly seem to be the main driving force for the speed of infection. Is that the case for the other two variants as well? Some suggestions of parameters to vary are given below under THINGS TO TRY.

Another thing that you may have noticed is that, in the "network" variant, there are cases where the disease will not spread to all people. This happens when the network has more than one components (isolated nodes, or groups of nodes that are not connected with the rest of the network) and that not all components get infected with the disease right from the start. NetLogo's network extension has a primitive that can help you identify the components of a network.

THINGS TO TRY

Set different values for the DISEASE-DECAY slider and run the ENVIRONMENTAL variant. How does the DISEASE-DECAY slider affect the results?

Similarly, set different values for the CONNECTIONS-PER-NODE slider and run the NETWORK variant. How does the CONNECTIONS-PER-NODE slider affect the results?

If you open the BehaviorSpace tool, you will see that we have a defined a few experiments that can be used to explore the behavior of the model more systematically. Try these out, and look at the data in the resulting CSV file. Are those results similar to what you obtained by manually playing with the model parameters? Can you confirm that using your favorite external analysis tool?

EXTENDING THE MODEL

Can you think of additional variants and parameters that could affect the spread of a disease?

At the moment, in the environmental variant of the model, patches are either infected or not. DISEASE-DECAY allows you to set how long they stay infected, but they are fully contagious until they suddenly stop being infected. Do you think it would be more realistic to have their infection level decline gradually? The probability of a person catching the disease from a patch could become smaller as the infection level decreases on the patch. If you want to make the model look really nice, you could vary the color of the patch using the scale-color primitive.

RELATED MODELS

NetLogo is very good at simulating the spread of epidemics, so there are a few disease transmission model in the library:

• HIV
• Disease Solo
• Disease HubNet
• Disease Doctors HubNet
• epiDEM Basic
• epiDEM Travel and Control
• Virus on a Network

Some communication models are also very similar to disease transmission ones:

• Communication T-T Example
• Communication-T-T Network Example
• Language Change

NETLOGO FEATURES

One particularity of this model is that it combines three different "variants" in the same model. The way this is accomplished in the code of the model is fairly simple: we have a few if-statements making the model behave slightly different, depending on the value of the VARIANT chooser.

A more interesting element is the Infection vs. Time plot. In the "mobile" and "network" variants, the plot is the same: we simply plot the number of infected persons. In the "environmental" variant, however, we want to plot an additional quantity: the number of infected patches. To achieve that, we use the "Plot update commands" field of our plot definition. Just like the "Pen update commands", these commands run every time a plot is updated (usually when calling tick). In this case, we use the create-temporary-plot-pen primitive to make sure that we have a pen for the number of infected patches, and actually plot that number:

if variant = "environmental" [ create-temporary-plot-pen "patches" plotxy ticks count patches with [ p-infected? ] / count patches ]

One nice thing about this NetLogo feature is that the temporary plot pen that we create is automatically added to the plot's legend (and removed from the legend when the plot is cleared, when calling clear-all).

HOW TO CITE

This model is part of the textbook, “Introduction to Agent-Based Modeling: Modeling Natural, Social and Engineered Complex Systems with NetLogo.”

If you mention this model or the NetLogo software in a publication, we ask that you include the citations below.

For the model itself:

• Rand, W., Wilensky, U. (2008). NetLogo Spread of Disease model. http://ccl.northwestern.edu/netlogo/models/SpreadofDisease. Center for Connected Learning and Computer-Based Modeling, Northwestern Institute on Complex Systems, Northwestern University, Evanston, IL.

Please cite the NetLogo software as:

• Wilensky, U. (1999). NetLogo. http://ccl.northwestern.edu/netlogo/. Center for Connected Learning and Computer-Based Modeling, Northwestern University, Evanston, IL.

Please cite the textbook as:

• Wilensky, U. & Rand, W. (2015). Introduction to Agent-Based Modeling: Modeling Natural, Social and Engineered Complex Systems with NetLogo. Cambridge, MA. MIT Press.

COPYRIGHT AND LICENSE

Copyright 2008 Uri Wilensky.

This work is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike 3.0 License. To view a copy of this license, visit https://creativecommons.org/licenses/by-nc-sa/3.0/ or send a letter to Creative Commons, 559 Nathan Abbott Way, Stanford, California 94305, USA.

Commercial licenses are also available. To inquire about commercial licenses, please contact Uri Wilensky at uri@northwestern.edu.

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