Diphtheria is a contagious, potentially fatal infection caused by toxin-producing bacteria of the Corynebacteriumdiphtheriae species complex, which includes C. diphtheriae, C. ulcerans, C. pseudotuberculosis, C. rouxii, C. belfantii, and C. silvaticum. Infection is localized principally in the upper respiratory tract, and production of diphtheria toxin (encoded by the tox gene) can cause systemic complications. This evidence concerns the gene TOX and infection.