That group of kids everybody knew in high school that individually were often the targets of derision and/or physical violence, but who through some geek radar found one another to form an alliance, thereby lessening the
probability that any one member is a victim.
The reference to the middle-age practice of isolating individuals with Hansen's Disease (Leprosy) and not allowing them to come into contact with any "normal" people is apropos because if a "normal" (read non-geekified) child were to associate with the leper colony, s/he is
immediately labeled as suspect, and in extreme cases will be isolated him/her self, thus becoming a member of
the colony.