The earliest example of the mathematical modelling of sheep disease? From Voltaire's Candide, 1759.
He was distressed, however, at having to part with his sheep, which he gave to the Bordeaux Academy of the Sciences. The Academy proposed as the theme for its yearly prize competition an investigation into the reason for the redness of the sheep’s wool. The prize was won by a scholar from the north, who demonstrated by A + B – C / Z that the sheep must of necessity be red, and furthermore that it would die of the rot.