"Smallpox blankets" have come up here a few times in the past:
https://www.reddit.com/r/AskHistorians/comments/3qslhq/where_smallpox_blankets_really_a_thing/
https://www.reddit.com/r/AskHistorians/comments/4qtnj4/how_widespread_were_smallpox_blankets/
tl;dr: there seems to have been one confirmed case of an effort to give smallpox-infested blankets to native Americans, by British forces during Pontiac's Rebellion in 1763.