But I began to ask questions and search out the usual things that I had been exploring before, like, what do the maps look like? What do the graphs look like? What does the data look like? And the maps of violence in most U.S. cities looked like this. There was clustering. This reminded me of clustering that we'd seen also in infectious epidemics, for example cholera. And then we looked at the maps, and the maps showed this typical wave upon wave upon wave, because all epidemics are combinations of many epidemics. And it also looked like infectious epidemics. And then we asked the question, well what really predicts a case of violence? And it turns out that the greatest predictor of a case of violence is a preceding case of violence. Which also sounds like, if there is a case of flu, someone gave someone a case of flu, or a cold, or the greatest risk factor of tuberculosis is having been exposed to tuberculosis.