Diphtheria, caused by toxigenic strains of Corynebacterium diphtheriae, is an 
ancient disease with high incidence and mortality that has always been 
characterized by epidemic waves of occurrence. Whilst towards the beginning of 
the 1980s, many European countries were progressing towards the elimination of 
diphtheria, an epidemic re-emergence of diphtheria in the Russian Federation and 
the Newly Independent States of the former Soviet Union demonstrated a 
continuous threat of the disease into the 1990s. At present, the epidemic is 
under control and only sporadic cases are observed in Europe. However, the 
circulation of toxigenic strains is still observed in all parts of the world, 
posing a constant threat to the population with low levels of seroprotection. 
More recently, Corynebacterium ulcerans has been increasingly isolated as 
emerging zoonotic agent of diphtheria from companion animals such as cats or 
dogs, indicating the enduring threat of this thought-to-be controlled disease.