Klebs-Loeffler bacillus is a type of bacteria that causes diphtheria in humans. There are various synonyms for this bacterium, including Corynebacterium diphtheriae, diphtheria bacillus, and Loeffler's bacillus. The bacterium was first identified by Friedrich Loeffler and Edwin Klebs in 1884. Diphtheria, caused by Klebs-Loeffler bacillus, is a potentially fatal disease which can lead to breathing difficulties, heart failure, and paralysis. Vaccinations are widely available to prevent the disease. Treatment generally involves the use of antibiotics and antitoxins to reduce the severity of symptoms. Early detection and intervention can greatly improve outcomes in individuals infected with the Klebs-Loeffler bacillus.