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Contagion Modeling in Plague Inc.

High density of land and air edges between countries.

The outbreak of a disease. One of the most credible threats to the longevity of the human species. James Vaughan of Ndemic Creations had a vision to make a game out of the potential extinction of mankind. Plague Inc. for iPhone (note: there is now a Plague Inc: Evolved for PC and Mac) puts players in control of a few key factors of a disease. The initially choose for their disease to be one of seven alternatives: a bacteria, a virus, a fungus, a parasite, a prion, a nano-virus, and a bio-weapon. After picking a starting state, for virtually all disease types, they now have no direct control of their fledgling plague. Diseases are grown indirectly by the player choosing to evolve certain mutations key to their strategy. There is a fine line between allowing the disease to be more contagious at the cost of symptoms being more apparent and keeping airports and docks open for all countries.

Vaughan’s game garnered so much popularity that the CDC actually covered it in their blog. The CDC actually asked Vaughan to share his modeling of outbreaks as they found it to be more realistic than theirs in some cases. Ali S. Khan writes that he “became interested in Vaughan’s game as a tool to teach the public about outbreaks and disease transmission because of how it uses a non-traditional route to raise public awareness on epidemiology, disease transmission, and diseases/pandemic information. The game creates a compelling world that engages the public on serious public health topics.” When asked about the hyper-realistic design at play, Vaughan responds, “Without a medical background, I did a lot of online research in order to make sure it felt realistic to players.  Luckily, I have always been very interested in biology as well as economics and current affairs.  This helped a lot when I was building the algorithms and models inside the game. A critical stage in the game is the ‘Infection Cycle’ that dictates how people become infected with a disease and how they infect others. The game revolves around this stage, and I spent months making sure that it worked properly. The core design is based on the concept of ‘basic reproduction rate’ and I found lots of great papers online which taught me more about it.”

Just what goes on in this infection cycle is elaborated upon in an article on The Escapist, “The Pathology of Plague Inc.” The article highlights the key elements that a player must consider for their disease to be successful: “Plague Inc. models how easily viruses and other pathogens can jump borders in today’s highly-connected world, where international air travel, transshipment and regional livestock markets can help spread a pathogen.” The ultimate deciding factor in the extinction of mankind? Whether the graph of the world’s countries is connected. If any country (I’m looking at you, Madagascar) closes all of its land borders, airports, and docks, the disease can only spread to it if there are enough animals alive to break the border (an extremely rare occurrence). The game also touches upon a few CDC nightmares by taking “world gatherings into account, meaning the World Cup or Olympics might spread the disease to a number of previously unaffected countries.” Events such as the Olympics creates new connections between humans that did not exist before. All it takes are a few carriers in the stands to infect the tens of countries from around the world watching the sports. This could be represented in a graph as many edges between people being formed temporarily, allowing the disease to spread. These ephemeral connections allow the disease to spread where it probably would have gone anyway, but allow for a much shorter path.

The disease can be described as an agent that attempts to make itself as widespread as possible on the planet. It seeks to travel across any edge from any node it is connected to, where we can define nodes to be either people or countries and edges as interactions between them (we could further defined weights for the edges corresponding to likelihood of contact). Having played the game myself, rural areas are harder to infect because of the rareness of encounters between people. On the contrary, densely populated urban areas can be devastated in a matter of days due to the highly connected nature of city life.

If you want to survive a pandemic, perhaps the wisest choice would be to become a hermit on an uncharted island in the middle of the ocean, making the world a disconnected graph and you a lone survivor.



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November 2014