I think it is safe to say that any action to detain or quarantine passengers would have to come from public health / law enforcement officials, acting in accordance with laws that permit them to do that sort of thing.

I think it highly improbable that a quarantine could be established prior to landing, in response to a threat which emerged in flight. Unless there was prior information on the threat, the responsible entities probably could not respond that quickly. There would be tremendous institutional inertia on the part of all parties involved to NOT detain a planeload of passengers...