Can Iratze heal only non-demonic injuries ?

[Gargs]

So, I started reading the book just today and I stumbled upon this term called "Iratze". I didn't know its meaning, so I looked it up on Google and found this -
http://www.shadowhunters.wikia.com/wi...
It says that Iratze is a healing rune but what bothers me is that its written that Iratze is used by Shadowhunters to heal them of non-demonic injuries. If its true, then why the author said in the prologue that Will could use a healing rune(Iratze) when he was stung by a demon's ichor and even got cut while fighting him ?

[Neha]

I think it is a mistake because Shadowhunters fight off demons and therefore, that is when they use iratze so that the don't have to heal the human way.