Left untreated, diphtheria can lead to:
Breathing problems: Diphtheria-causing bacteria may produce a toxin. This toxin damages tissue in the immediate area of infection — usually, the nose and throat. At that site, the infection produces a tough, gray-colored membrane composed of dead cells, bacteria and other substances. This membrane can obstruct breathing.
Heart damage: The diphtheria toxin may spread through your bloodstream and damage other tissues in your body, such as your heart muscle, causing such complications as inflammation of the heart muscle (myocarditis). Heart damage from myocarditis may be slight or severe. At its worst, myocarditis can lead to congestive heart failure and sudden death.
Nerve damage: The toxin can also cause nerve damage. Typical targets are nerves to the throat, where poor nerve conduction may cause difficulty swallowing. Nerves to the arms and legs also may become inflamed, causing muscle weakness.
If the diphtheria toxin damages the nerves that help control muscles used in breathing, these muscles may become paralyzed. At that point, you might need mechanical assistance to breathe. With treatment, most people with diphtheria survive these complications, but recovery is often slow.
Courtesy: Mayo Clinic