I want to give someone (a lot of someone, really) a cold (because I'm an evil wizard and I can).
At this point, nothing too serious, say an Ease Factor 6 doing a Light Wound. What would be a reasonable Base spell level for that? I'm thinking since the target gets a resistance check (as per debillitation on page 180) 4 would be a good start. Sound good?
And while I'm at it, what would be a good guideline for more powerful diseases? And additional magnitude for +3 Ease Factor as for CrAq guidelines?
Any particular reason why you are giving them a chance to resist? You are affecting the target's body directly, so you can just cause the disease. Their resistance, if any, is the magic resistance.
But if you really wish to add resistance rolls to Perdo Corpus diseases, I would start the Ease Factors from 3, just to stay in line with those Creo Aquam guidelines, then adding +3 for each magnitude. But to be honest, I don't think is really necessary or particularly useful.
Aren't colds Evil spirits and Humors? Doesn't this mean that you would be 'attacking' people with demons?
Because I want it to appear to be a natural dissease. I don't want to kill people outright... I just want them to be miserable so they will leave, hopefully without becoming suspicious to supernatural causes.
The rules on disease in the deprivation section talks about the humors without mentioning disease spirits, so I take that to mean that while some diseases could be caused by spiritis, not all are presumed to be.