What we need to realize is that we need to be able to compute c² = sqrtb²a²

I present case tree plots and checkerboard plots for visualizing contagions. The visualizations are best suited for diseases like SARS, MERS-CoV and H7N9 for which there are a limited (less than 200) number of cases, with data available on human to human transmission. They a) allow for easy estimation of epidemiological parameters like basic reproduction number b) indicate the frequency of introductory events, e.g. spillovers in the case of zoonoses c) represent patterns of case attributes like patient sex both by generation and over time.