It would only be genocide if you claimed that every person who shares their race, ethnicity, religion, nationality, etc etc, also had that disease and they all needed to die. If you're targeting just a small area, like a town, or a building, regardless of the race, ethnicity etc etc of the people in there, that's a quarantine.
Re: Is there anything that's beyond the pale?
It would only be genocide if you claimed that every person who shares their race, ethnicity, religion, nationality, etc etc, also had that disease and they all needed to die. If you're targeting just a small area, like a town, or a building, regardless of the race, ethnicity etc etc of the people in there, that's a quarantine.