Over the years, some of the most frequent animal warriors have been the smallest. Chucking bee hives over the wall of a castle is only the tip of the iceberg when it comes to using insects as weapons of war. One of the mightiest threats was also the tiniest: the flea, which carries the bacteria Y. pestis—source of the plague. Carried by fleas living on the skin of infected rats, Y. pestis is the source of the infamous Black Death. In fact, some historians suggest that the epidemic reached Europe when Mongols besieging the town of Caffa threw plague-infected dead bodies into the town.
It took more than five hundred for scientists to isolate the bacteria and discover the role fleas played in infection, but once they did it wasn’t too long before countries started looking at using fleas to deliberately infect their enemies with the plague.
In World War Two Japan, the biological warfare group Unit 731 experimented with using spreading infected fleas by dropping them from aircraft. Unit 731 did some truly horrifying things to its human research subjects, so there’s no surprise that they were willing to test their weapon—called the Uji bomb and holding 30,000+ plague fleas—on nearby Chinese towns. By the time the war ended Unit 731 had moved on to planning ways to drop the flea bombs on American cities, either by plane or balloon.
On the Allied side, plague flea research took place in Canada. Working at the old quarantine station of Grosse Île, Canadian scientists experimented with using fleas to pass plague and typhus as part of a joint UK-US-Canada collaborative biological warfare project.
While those weapons were never released, after the war the US continued to toy with fleas as a way of getting a bio-weapon to the target. In Operation BIG ITCH, hundreds of thousands of uninfected fleas were dropped in flea bombs to test the dispersal and survival rates of the insects.
By the time the Biological Weapons Convention came into effect in 1975, science had passed the plague flea by. Whatever biological weapons exist in arsenals today are probably far more sophisticated and far more deadly than a batch of flying fleas.