Wired: Determining the origin of a bit of information, or of a disease outbreak, has often required the backwards, step-by-step tracing of the transmission. However, a new process can calculate the most likely source using only a fraction of the information required by earlier methods. Researchers at ETH Zürich recently estimated, and then combined, the most likely paths of transmission to individual nodes within a network. When they applied their technique to a known cholera outbreak in South Africa in 2000, the researchers could narrow down the source by using information about the presence of cholera in just 20% of the network’s nodes — in this case, communities in the region. The same technique was used to determine the identity of the leader of a terrorist group and the spread of contamination in a subway system.