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.
While the small pox/disease ridden blankets may or may not be factual, there is plenty of historical records predating Europeans landing in the Americas of using disease in warfare.
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.
There are well documented instances and for example vaccination against smallpox was found a mere 35 years in 1798 after smallpox ridden blankets were intentionally used at the siege of Fort Pitt in 1763. And inoculation predates that so the disease itself was reasonably understood if not in a particularly accurate to reality manner.
