Anyone who has ever met a Hell's Tuna is a Hell's Tuna, whether or not the person realizes it. Anyone that person then meets also becomes a Tuna. Anyone who pretends to be a Hell's Tuna without our recognition is in big trouble.
Many people are Tunas without even knowing it.
They may either have unknowingly had contact with a Hell's Tuna, or through some quirk of fate been born into this state. A prime example of this is Mrs. Banes.