Sometimes I say "bless you" and sometimes I say "gesundheit", and sometimes I say, "whatever you got, get it away from me". Totally depends on the situation and the person. I never say it just out of habit, or because I think I'm exorcising demons, or even because I'm afraid the person may be ill. But a sudden sound like a sneeze makes a person alert and notice the sneezer, so I take the opportunity to let the person know that I care about them. Seems logical enough to me.