Fig. 1

Four phases of the epidemic curve based on the theory of biological invasion (The five phases include transport, colonization, establishment, landscape spread, outbreak. The transport phase is define as the time of reporting of the first case in Additional file 1: Fig. S1. Each phase was partitioned by the segmentation of the theoretical curves. The newly reported cases of each city were illustrated in blue bar. The case distribution patterns in each city is also showed under the figures to corresponding to each epidemic phase)