Fleas will come and go without blood to drink. That doesn't mean there can't be mites of some sort.

"Yes, it's hypocritical, isn't it? Humans get like that," Toby says, student of a bookish middle school dropout. "They often hold creeds of acceptance and fellowship while also doing terrible things to people who are only slightly unlike them. The Mirrorbound are more unlike them. Even the humans have no older ties to these place and these people."