| 要旨 |
A long lasting question in epidemiology is why many childhood diseases show periodicity of more than one year. It is well known that the conventional epidemiological model with seasonally varying transmission rate shows the muti-year periodicity depending on the strength of seasonal forcing. We studied the evolution of seasonality using the compartment model.A strain of influenza viruses, for example, would be able to improve the transmission rate in summer, thereby reducing the degree of seasonality in transmission rate. Another influenza strain may improve the transmission rate in winter in expense of the reduced transmission rate in summer, thereby exaggerating the seasonal difference in transmission rates. Hence the seasonality itself (or sensitivity to the seasonality) is subjected to evolution. It is believed that larger fluctuation in an environment is not favored in the life history evolution because it reduces the geometric mean fitness; however, we show that an opposite can be true -- the evolution favors larger fluctuation in a transmission rate whereas it reduces the geometric mean fitness of the basic reproductive ratio. By applying our new principle, we can answer why many childhood disease show multi year periodicity.
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