There does not seem to be any global difference in epidemiology. The overall age- and sex-adjusted annual incidence rate calculated for one state in the US was estimated to be 4.9 cases per 100,000.[2] The female-to-male ratio was 3.5:1, with most cases occurring between the ages of 30 and 50 years.[2] A seasonal variation has been noted in several studies in many countries, with the highest incidence in the summer and autumn in the northern hemisphere (July to December), which is typical for viral infections.[7]

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