It's a non-contagious disease. They cannot catch it or pass it on. They either all have it by being born with it or developing it such as cancer or heart disease or none has it except the one identified by the abbot. There being only one infected monk when the Abbot returns he finds that only one monk killed himself.He might have been identified by the abbot as "the one" or seen a reflection, or all the others might have run away when they saw it. (And presumably the other monks were all ok)

leastways the pivot is that the disease is non contagious.

At least it's wot I fink......