I think I missed when they explained this in the game. Why do we kill hiss-infected people instead of containing/cleansing them?

if >!jesse can purge the hiss out of stuff!<why do we kill infected people, even named characters? i guess they are so mutated that even if we could get the hiss out, they would still be mutants? what about the floating chanters? Characters behave like it's totally impossible to get hiss out of stuff but we cleanse control points, altered items, and >!dylan!<. I thought the difference with >!jesse and dylan is polaris helping fight off the hiss. Dylan doesn't have polaris's voice in his head anymore, so jesse being able to cleanse him is just because she's directing polaris to do it.!<