I don't think it survives in the environment, and it doesn't seem to have any animal hosts. There are places in the world where it's endemic and somewhat common, and it can live in the pharynx of vaccinated or asymptomatic humans. So it probably comes into a country from an immigrant or traveler with some frequency, it just doesn't spread because of vaccination.
Then there's this kid.
From microbewiki (emphasis added): "C. diphtheriae is a Gram-positive, aerobic, nonmotile, toxin-producing, rod-shaped bacteria belonging to the order Actinomycetales, which are typically found in soil, but also have pathogenic members such as streptomyces and mycobacteria."