Background: Dengue fever/dengue haemorrhagic fever is prevalent in Thailand, where all serotypes are found and the dominant serotype has changed irregularly. Although almost all primary infections present with slight symptoms or are asymptomatic, little is known about the infectiousness of dengue fever. Methodology: A mathematical model of the transmission for dengue virus was constructed covering the possibility of sequential infections with all four different serotypes. The model was combined with the seasonal population dynamics of Aedes aegypti, the principal vectors of dengue virus in Thailand. The contributions of inapparent cases in the transmission to mosquito vectors and antibody-dependent enhancement were incorporated into the model. Moreover, the hypothesis of an "unnatural" infection route was examined, where a person acquires immunity by infection during a cross-immunity period, through model simulations. Results: A comparative study on the transmission probabilities of inapparent cases to mosquito vectors showed that the prevalence of dengue infection could be immediately stamped out after a severe outbreak if inapparent cases had no infectiousness. The simulation under an "unnatural" infection route assumption resulted of yearly changes in the dominant serotype and sharp, irregular variations in outbreaks. Conclusion: The supposition that inapparent cases had no infectiousness was not in accord with the actual situation in Thailand. Furthermore, the simulation result supported the "unnatural" infection route as having an influence on epidemics of dengue.