Thus it is rare and hopefully will never re-appear, but we keep it just in case it does...Therefore "rare" is a better term for it than "extinct"...
Polio is rare, Cholera is rare, leprosy is rare
Pox is eradicated since 1979. The fact that the virus is exsiting in laboratories, but never showed it's ugly face since 79, even for one case, don't fits the definition of rare.