A little-known effort to conduct biological warfare occurred during the 17th century. The incident transpired during the Venetian–Ottoman War, when the city of Candia (now Heraklion, Greece) was under siege by the Ottomans (1648–1669). The data we describe, obtained from the Archives of the Venetian State, are related to an operation organized by the Venetian Intelligence Services, which aimed at lifting the siege by infecting the Ottoman soldiers with plague by attacking them with a liquid made from the spleens and buboes of plague victims. Although the plan was perfectly organized, and the deadly mixture was ready to use, the attack was ultimately never carried out. The conception and the detailed cynical planning of the attack on Candia illustrate a dangerous way of thinking about the use of biological weapons and the absence of reservations when potential users, within their religious framework, cast their enemies as undeserving of humanitarian consideration.