Diphtheria is a forgotten disease that, despite advances in medicine and vaccination, is increasingly re-emerging in various parts of the world, including developed countries. It is caused by the bacterium Corynebacterium diphtheriae, which produces diphtheria toxin responsible for the severe course of the disease. However, only toxigenic strains-those carrying the tox gene, are capable of producing this toxin. The majority of C. diphtheriae strains isolated in Poland do not contain this gene, which is why infections caused by them are usually milder. Depending on the site of infection, symptoms most commonly involve the upper respiratory tract, with the hallmark sign being the presence of pseudomembranes. Rapid diagnosis and accurate confirmation of the pathogen are essential not only for implementing proper preventive measures, but above all for initiating timely treatment and preventing serious organ complications. Vaccination plays a key role in diphtheria prevention, offering effective protection against infection and transmission. Despite the availability of both preventive and therapeutic measures, diphtheria is becoming an increasing threat in countries with low vaccination rates. The growing number of cases particularly among children, highlights the urgent need for education and improvement in both prevention and treatment strategies.