You're wrong here on many points, not least of which, people who got banished to leper colonies included people with generic skin conditions - completely for this exact reason: they didn't know what causes leprosy, so they quarantined ANYONE WHO POSED A RISK OF TRANSMISSION.
That's what quarantine is AND HAS ALWAYS been. The only reason you believe otherwise is because you live in a place where this hasn't happened in a really long time.