Diphtheria signs and symptoms usually begin two to five days after a person becomes infected and may include:

• A thick, gray membrane covering your throat and tonsils
• A sore throat and hoarseness
• Swollen glands (enlarged lymph nodes) in your neck
• Difficulty breathing or rapid breathing
• Nasal discharge
• Fever and chills
• Malaise

In some people, infection with diphtheria-causing bacteria causes only a mild illness — or no obvious signs and symptoms at all. Infected people who remain unaware of their illness are known as carriers of diphtheria, because they can spread the infection without being sick themselves.

Skin (cutaneous) diphtheria

A second type of diphtheria can affect the skin, causing the typical pain, redness and swelling associated with other bacterial skin infections. Ulcers covered by a gray membrane also may develop in cutaneous diphtheria.

Although it's more common in tropical climates, cutaneous diphtheria also occurs in the United States, particularly among people with poor hygiene who live in crowded conditions.

The bacterium Corynebacterium diphtheriae causes diphtheria. Usually C. diphtheriae multiplies on or near the surface of the mucous membranes of the throat. C. diphtheriae spreads via three routes:

• Airborne droplets. When an infected person's sneeze or cough releases a mist of contaminated droplets, people nearby may inhale C. diphtheriae. Diphtheria spreads efficiently this way, particularly in crowded conditions.
• Contaminated personal items. People occasionally catch diphtheria from handling an infected person's used tissues, drinking from the infected person's unwashed glass or coming into similarly close contact with other items on which bacteria-laden secretions may be deposited.
• Contaminated household items. In rare cases, diphtheria spreads on shared household items, such as towels or toys.

You can also come in contact with diphtheria-causing bacteria by touching an infected wound.

People who have been infected by the diphtheria bacteria and who haven't been treated can infect nonimmune people for up to six weeks — even if they don't show any symptoms.

People who are at increased risk of contracting diphtheria include:

• Children and adults who don't have up-to-date immunizations
• People living in crowded or unsanitary conditions
• Anyone who travels to an area where diphtheria is endemic

Diphtheria rarely occurs in the United States and Western Europe, where health officials have been vaccinating children against the condition for decades. However, diphtheria is still common in developing countries where immunization rates are low.

In areas where diphtheria vaccination is standard, the disease is mainly a threat to unvaccinated or inadequately vaccinated people who travel internationally or have contact with people from less-developed countries.

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, showing up as minor abnormalities on an electrocardiogram, or severe, leading 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 C. diphtheria toxin damages the nerves that help control muscles used in breathing, these muscles may become paralyzed. Respiration may then become impossible without a respirator or another device to assist with breathing.