On a purely curious note, are there people in this world that cannot use magic?

I know you've mentioned that it's a natural aspect of life, but there often seems to be exceptions to this rule. I mean, there are people who do have a harder time breathing than others, and babies sometimes forget(or just don't) to do it. And even in Harry Potter, is Squib the term I want? What was Filch? Anyway! Are there people like that around? And is that considered weird, or just as normal as someone who uses magic? What might the ratio be if this is a naturally occuring phenomenon? Would a coven accept those people into their group/home/family? 

Just wondering how far the variation in capabilities might go between doing something exceptional and not being able to do anything at all, at least as far as magic is concerned. Or if the sliding scale stops before it cuts anyone off from this particular ability.