"The earliest documented incident of the intention to use biological weapons is possibly recorded in Hittite texts of 1500–1200 BC, in which victims of tularemia were driven into enemy lands, causing an epidemic" [1].

Smallpox is a can of worms. But if you learn your enemy are idiots and don't have a flu vaccine, and you know your own forces do, I don't think you're going to get yourself sanctioned by your trade partners for exposing them to the flu. It will take out a statistically-measurable fraction of their troops, at a predictable interval, something you can plan to exploit with manoeuvre.

[1] https://en.wikipedia.org/wiki/History_of_biological_warfare#...