Epidemiology
The overall age- and sex-adjusted annual incidence rate calculated for one county in the US was estimated to be 4.9 cases per 100,000.[2] The female-to-male ratio is 3:1, with most cases occurring between the ages of 30 and 50 years.[7] A seasonal variation has been noted in several studies in many countries, including the US, with the highest incidence in the summer and autumn in the northern hemisphere (July to December), which is typical for viral infections.[8]
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