If indeed it could be called an illness, it is not the same in scope as any illness that has ever existed. It's an affliction that's gone global and that threatens to make humanity extinct.
You can't really compare the barn to a mental hospital because even those in there can still reason, speak, and aren't constantly trying to eat other people unaffected by their "disease". And even if they were trying to, if they greatly outnumbered the sane population, you can bet that some killing would happen.
Yeah it should be clear to everyone in that world by now that whatever's causing this, isn't based on any kind of real world science. This isn't just an advanced form of rabies they're dealing with; these are supernatural creatures which can survive and function in ways that are simply not possible.
Not genuine real world science, but it seems to me it's supposed to be science-based in TWD universe -- albeit not yet understood science.