Does diphtheria have a high mortality rate?

Toxic strains 3-23%: Diphtheria has more & less toxic strains with impact based on toxicity. In unprotected areas the case fatality rate during epidemics in the 90's was 3-23%. See aap/red book/28ed/diphtheria.

If untreated. : In the 1920s, there were an estimated 100, 000 to 200, 000 cases of diphtheria per year in the United States, causing 13, 000 to 15, 000 deaths per year in cases that progress beyond a throat infection, diphtheria toxin spreads through the blood and can lead to potentially life-threatening complications that affect other organs. In these severe cases, about 40% to 50% of those left untreated can die.