Ternos

Morbis quoque enim quasdam leges natura posuit: quadrini circuitus febrem numquam bruma, numquam hibernis mensibus incipere, quosdam post sexagensimum vitae spatium non accedere; aliis pubertate deponi, feminis praecipue; senes minime sentire pestilentiam. namque et universis gentibus ingruunt morbi et generatim modo servitiis, modo procerum ordini aliosque per gradus. qua in re observatum a meridianis partibus ad occasum solis pestilentiam semper ire nec umquam aliter fere, non hieme, nec ut ternos excedat menses.
(Pliny the Elder, Nat. Hist. 7.170)

For nature has imposed certain laws even upon diseases: a four-day-period fever never begins at mid-winter or in the winter months, and some people are not attacked by it when over the age of 60, while with others, particularly women, it is discarded at puberty; and old men are least susceptible to the plague. For diseases attack not only entire nations but also particular classes, sometimes the slaves, sometimes the nobility, and so through other grades. In this respect it has been observed that plague always travels from southern quarters westward and almost never otherwise, and that it does not spread in winter, nor during a period exceeding three months. (tr. Harris Rackham)