I have to admit that I thought biological warfare was a 20th-century invention, but after a little research, I have discovered that it has been around for quite some time.
During the Revolutionary War, Brigadier General Jethro Sumner wrote to Major General Horatio Gates to inform him that:
“A Woman who passed about 15 days ago I am apprehensive has proceeded towards Hillsborough, from the enemy, with the small-pox. I doubt not but she has been sent on purpose to spread that Contagion among the Troops. The Ferry man at this ford, I am informed by Doctor Pasteur & Alexander, whom I sent to examine him, has got it. I have had him removed, & shall take such precaution in my power to prevent the infection spreading.”
[Sumner to Gates, 8 October 1780. Colonial and State Records of North Carolina, Vol. XIV, pp. 678-679]