Reverse Epidemic Diffusion

In Wired magazines “Finding the Sources of Epidemics,” Samuel Arbesman discusses how networks can be used to find the source of an epidemic using as few as 20 percent of that network’s nodes. According to Mr. Arbesman “they (Pedro Pinto and his team) explore how the leader of a terrorist organization can be identified and even show how this methodology could be used in finding contamination sources in a subway system. They found that they could determine the source of a contamination to within a single subway stop by monitoring the behavior fewer than 20 percent of the stations.”

In this case, reverse diffusion was used to find the source of a cholera outbreak in S. Africa after the fact. However, models are one thing and their applications are quite another. In all of these examples except one, reverse diffusion is useful for finding a static hub. But no evidence backs the claim that the leader of a terrorist organization could be caught by working backwards with statistics. For one thing, the leader might move, making he or she harder to pinpoint than the proverbial hot zone subway stop this author mentions. Secondly, the leader might operate through proxy. Finally, reverse diffusion requires that 20 percent of the nodes in the network be analyzed. The last requirement is the most debilitating, because with sick people, the researcher can deal in absolutes – this person is ill and this one definitely is not. But how could this method be applied to something as complex as terrorism, where the researcher can’t be sure if their nodes are the right nodes? In terrorism, the researcher has to diagnose degrees of ideological infection, which is a far more abstract concept than a viral epidemic.