I've heard stories of smallpox riddled blankets being gifted on purpose. But I suspect if there's something to that, it came later when we had a better idea of how disease spread.
When sieging cities in antiquity water supplies would be contaminated. Arrow heads be smeared with poison and otherwise feces to cause infections. In Europe during Middle Ages corpses and feces of diseased would be catipulted into cities to cause infection, notably this this even happened with corpses that died of the bubonic plague.