Any one of thousands of different microorganisms that affect the health and safety of the world's populations of humans, animals and plants could potentially be weaponised; that is undergo research and development whose aim is to create species or strains that could serve as deadly payloads for spray systems, bombs, rockets or missiles. However, many historical studies of warfare have made it clear that only a few species of bacteria and viruses have been weaponised. As is made clear in this paper, of the pathogens weaponised in the 20th century by Japan, the Union of Soviet Socialist Republics (USSR), the United States and Iraq, most were zoonotic pathogens. If a nation or terrorist group were to acquire biological weapons in the future, it is most likely that their payload would be a zoonotic pathogen.