Let us consider a medieval word without magic (there are castles, knights, kings, blacksmithing, sail ships, monotheistic faith) very close to ours. On one unlucky day the borderland deserts are crossed by unfriendly nomads (like medieval Mongols). These nomads raid cities and wreak havoc. The most terrifying part about them is they are carriers of some deadly disease like Typhoid Mary. They have this disease, and they do not suffer from it, and everyone else - yes, even the breath or blood of a nomad can spread contagion.
So, the question is: what can this disease be and how can this wicked protection of nomads be possible?
I leave my thoughts here;, I'll be grateful for your ideas and help.
Hint 1: The nomads' immunity can be result of generations of natural selections, and the real carriers of the disease can be, for example, fleas. They live near nomads for generations, and only the immune one nomads survived.
Hint 2: The shamans of nomads knows the secret cure, and they distribute it among their kin along with the meat of diseased animals.