  The epidemic spreading of a disease can be described by a contact network
whose nodes are persons or centers of contagion and links heterogeneous
relations among them. We provide a procedure to identify multiple sources of an
outbreak or their closer neighbors. Our methodology is based on a simple
spectral technique requiring only the knowledge of the undirected contact
graph. The algorithm is tested on a variety of graphs collected from outbreaks
including fluency, H5N1, Tbc, in urban and rural areas. Results show that the
spectral technique is able to identify the source nodes if the graph
approximates a tree sufficiently.
