Smallpox was weaponized and used prior to 1776. Though never as wide-spread as popularly believed, items from the infirmary believed to be infected with smallpox were given as gifts during the Pontiac rebellion. 

William Trent, Commander of the militia in Pittsburgh wrote in his journal in May of 1763, "... we gave them two Blankets and an Handkerchief out of the Small Pox Hospital. I hope it will have the desired effect." 

Other contemporaneous correspondence also supports the practice.