"C. diphtheriae" is a Gram-positive bacterium that causes diphtheria, a severe respiratory disease that is primarily spread through coughing and sneezing. Some synonyms for "C. diphtheriae" include Corynebacterium diphtheriae, Klebs-Löffler bacillus, and diphtheria bacillus. Scientists and medical professionals use these different terms to refer to the same bacterial species, which has a unique shape and features that enable it to cause illness in humans. Diphtheria is a rare disease today thanks to widespread vaccination efforts, but it remains a serious threat in some parts of the world where access to vaccines and healthcare is limited.