The respiratory form has an incubation period of 1-4 days. The onset of diphtheria is usually gradual. Symptoms include

 Sore throat may range from mild to severe with painful swallowing and hoarseness of the voice.

 Drooling which indicates the obstruction of the airway passage.

 Fever and chills.

 Bloody, watery serosanguineous drainage from the nose.

 Croup-like coughing.

 Stridor, difficulty in breathing or sometimes-rapid breathing.

 Apnea may occur.

 Bluish coloration of the skin may be caused due to lack of oxygen.

 Skin lesions can be seen in cutaneous diphtheria.

 Swollen glands due to the enlarged lymph nodes in the neck (bull neck).

A hallmark sign is that the bacteria produces toxin that can lead to a thick, gray covering in the nose, throat or airway, which separates it from other respiratory illnesses. This covering is usually fuzzy gray or black and causes breathing difficulties and painful swallowing.

The toxin causes tissue damage (necrosis) in the immediate area of the infection, usually the nose and throat. Toxin can also spread via the bloodstream to other organs, where it can cause significant damage. Although the toxin can damage any tissue, the heart, kidney and nervous system are most frequently and most severely affected.

In mild case of the illness the patient shows no signs or symptoms of the disease. They are carriers but they can be contagious. If the infection is not detected in the early stage and is left untreated it results in cardiomyopathy and peripheral neuropathy.