Yes, it does sound like a medieval incantation of doom. This is one for you from Nature’s Book of Bad Dreams.
Seems we have among us a fungus that infects periodic cicadas in a most bizarre and ghoulishly effective way. So if you find said insect with its back half white, it is a flying salt-shaker of death to others of its kind. It will not turn you into anything more or less than what you have been. OTOH…
The white pustule is a teeming mass of fungal spores, that, when germinated in the tissues of a passing cicada, turn it into a living zombie, roaming the world for weeks with its hind-most parts replaced by the fungus that has eaten it alive. Note that the fungus, too, has periodicity so to be timed to “bloom” along with the emergence of its meal and meal ticket.
During its zombification, the chances of spores finding other victims are increased by the fact that chemicals are released by the spores (including a couple of known human hallucinogens, but you’d have to eat a bucket full of insects to get a, er, buzz.) These drugs induce male cicadas to produce sounds typical of females to lure in other males not yet infected. And also infected males respond by seeking out calls of both sexes, thereby increasing the odds of contact and contagion.
Order it by name: Massospora cicadina