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K - College of Natural Resources - University of California, Berkeley

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observations for the diseases examined here. Furthermore the behavior <strong>of</strong> mean-based<br />

models can differ sharply from that predicted when data-driven levels <strong>of</strong> variation are<br />

incorporated (Figures 3B-C), and from observations in the field: real outbreaks <strong>of</strong>ten<br />

grow too fast, or die out too frequently, compared to predictions <strong>of</strong> models assuming<br />

homogeneous R0. We argue that a continuously-distributed individual reproductive<br />

number, ν, is a logical and necessary extension to the concept <strong>of</strong> R0. Data show that<br />

individual variation is a universal feature <strong>of</strong> disease transmission (Tables 1,2,S1,Figures<br />

1B-C,2A-B,4C (Woolhouse et al. 1997)), if not always in a fixed 20/80 proportion, and<br />

epidemiological theory should reflect that reality.<br />

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