As several cases were reported in Italy among travelers returning from endemic areas [46] (only one, however, in the study area; additional imported cases throughout the duration of the outbreaks were not detected), the question arises why no previous outbreaks of CHIKV occurred in other Italian regions. By assuming B = 200 ha {1 , we estimated the probability p (see Eq. 9) that a major outbreak of the disease would occur after the introduction of a single infective host to be 0.59 (95% CI 0.35-0.76, see Fig. S5 ) and, by assuming the same density of mosquitoes, epidemic outbreaks are more likely in rural areas with respect to urban areas -as the human population density is much lower in the former. This could explain why cities like Cesena (96,000 inhabitants), Rimini (141,000 inhabitants) and Bologna (377,000 inhabitants) and Ravenna (157,000 inhabitants) located in the same region of the two most affected villages did not experience any epidemic outbreak, though sporadic CHIKV cases were recorded in the same period [1, [42] [43] [44] . These results support the hypothesis that outbreaks of Chikungunya virus in those temperate climate countries characterized by high density of Aedes albopictus are probable after the importation of an index case from abroad.