Impact of population size on incidence of rubella and measles in comparison with that of other infectious diseases.

A strong dependency of rubella and measles epidemics on population size was confirmed by 2 types of plots: the cumulative frequency distribution of number of cases per prefecture and the slope of the log-log plots of number of cases per prefecture on the y-axis vs. prefecture population size on the x-axis. These parameters were found to be constant and unique to each infectious agent. The broader cumulative frequency distribution and steeper slope of the log-log plots were characteristic to measles and rubella, i.e., a higher population size was correlated with a disproportionate high incidence of measles and rubella. No such tendency was found in other infections with possible exceptions of pertussis and keratoconjunctivitis. The dependency of rubella and measles on population density was examined by log-log plots of patient number/population vs. population density, which revealed strong population density dependency of rubella; the dependency of measles on population density was equivocal.