In my world, magic is an illness that you get by getting infested with a virus, think rabies. As such, certain animals are hunted and killed so nobody can get infested.
Once you are infested, you know you'll die soon.(months to years), but it is coming. It is possible to lengthen your lifespan through potions/alchemy but it remains inevitable. However, the longer you are infected the stronger you become. But the longer you live, the bigger the effects are(restless/aggressive behavior, pain when drinking...).
The one thing for which I need a solution is with off springs of the infected. I was thinking of giving them the permanent ability but then in a weakened form.
Take note that I don't count shamanism and the likes as magic...
What do you think about it? I know it's rather basic but I like 'low magic'. But it's plain and simple to understand...
Life is a sexually transmitted disease that is invariably fatal. Animal life, that is; who cares if a potato is immortal, cloning itself down the generations or a starfish can reproduce vegetatively and asexually indefinitely?
How much power does magic give, when contracted? There might be men who would deliberately infect themselves to obtain vengeance, or Kings who maintain a reservoir of infected animals in order to generate magic berserkers on demand, if the powers gained are sufficient. Sort of like fantasy suicide bombers. And the enemy would know that slaying these will give a high probability of contaminating others and spreading the disease, into both sides' troops. How many of your soldiers would, if aware that they were condemned anyway, continue to obey orders and attack mainly the enemy?
But very few mediaeval diseases were caused by organisms, even invisible demons; most were from curses, Karma and disapproval by superior beings, some gods, some go betweens. Insufficient sanitation had nothing to do with it, unless the Gods had decreed latrine digging. This is more like a demon infestation.