Seasonality of plague in early modern Europe: Swiss epidemic of 1628--1630.

The course of plague is markedly influenced by seasonal factors. A quantitative estimate of its seasonal distribution was made from parish death books of 38 communities in a small, diversified area of Switzerland for the epidemic period of 1628--1630. The temporal distribution of outbreaks was bimodal with maximal mortalities in the autumn-early winter seasons of two successive years. Of 43 peaks of mortality, 39 occurred between September and January; November was the month of highest frequency. Colder weather exerted a gradual limiting influence, but late-blooming community outbreaks continued at a high or maximal level as late as January. Evidence is presented that the temporal distribution of outbreaks affected the persistence of plague in the region. The seasonality of plague in Switzerland contrasts with the pattern in Great Britain, where midsummer epidemics prevailed.