What method of germ warfare did the Mongols use to spread the plague to Italian merchants on the Crimean Peninsula?

Biological warfare.

Explanation:

Mongol Empire used biological warfare method (black plague) to kill humans in the Crimean peninsula. Mongol troops threw plague victims into the city, thus contaminating the inhabitants. The Mongols were the first to use germ warfare. The Mongol army threw infected bodies into the attacked Crimean city of Caffa, and that led some of the survivors to flee and spread epidemic from Caffa to the Mediterranean Basin.

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