The epidemic spreading of a disease can be described by a contact net-work whose nodes are persons or centers of contagion, and links are het-erogeneous relations among them. We provide a procedure to identify multiple sources of an outbreak or their closer neighbors. Our method-ology is based on a simple spectral technique that requires only the definition of a undirected contact graph. The algorithm is tested on a variety of graphs collected from real influenza outbreaks, both in urban and rural areas. The proposed spectral technique is able to identify the source nodes in cases when the graph sufficiently approximates to a tree. © 2014 Vincenzo Fioriti et al.

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