Frounce is a highly contagious yeast infection of the digestive tract. Frounce is caused by a protozoan called Trichomonas which is frequently present in the crops of pigeons. For this reason, pigeon heads and crops are generally not fed to raptors. The typical signs of frounce are white spots in the mouth or crop, often described as "cheesy" or "white plaques."
Picture from here
Frounce is very often deadly to falcons if it is not treated quickly. Transmission happens through bedding or beak to beak contact in dove species and falcons are infected when they eat pigeons. DNR tells it this way:
Transmission of T. gallinae occurs by discharge of bodily fluids in one of four ways. Adult doves and pigeons infect their offspring during feeding, infect other adult birds through contaminated food, water, and bedding and via courtship behavior. Raptors are infected through consumption of infected doves or pigeons.
This is a disease primarily of doves and pigeons and is transmitted from the adult to their offspring by the regurgitational method of feeding used by these birds.
But this disease is not only deadly to raptors today (I'm on a binge here). It has been theorized that Frounce has been around much longer than previously thought. Scientists believe that many parasites develop along with their hosts, so today's bird parasites, were yesterdays dinosaur parasites. An article for National Geographic News states:
After surviving countless battles, a giant T. rex was ultimately taken down by a microscopic parasite akin to one carried by modern pigeons, scientists say. The finding is a new interpretation of multiple holes in the jawbone of "Sue," the largest and most complete Tyrannosaurus rex fossil yet found, which is on display at the Field Museum in Chicago.
And goes on to say....
In a new study, researchers instead propose that the holes are lesions made by an ancient version of trichomonosis, a single-celled parasite that infects the throats and beaks of modern birds.