What Is the Disease Diphtheria and Which Part of the Body It Affects

Diphtheria is a serious, potentially life-threatening bacterial infection caused by Corynebacterium diphtheriae. It most commonly affects children between the ages of one and five, though adults can also contract it. Infants under one year old — especially in their first months of life — become ill far less often, largely because of protective antibodies passed from an immunized mother. The disease is now rare in the United States and other countries with strong vaccination programs, but it remains a threat wherever immunization coverage is low.

What is the disease called diphtheria?

Diphtheria is an acute infectious disease in which a toxin-producing bacterium colonizes the mucous membranes — most often of the throat — and releases a poison that damages tissue locally and can spread through the bloodstream to injure the heart, nerves, and kidneys. The hallmark of respiratory diphtheria is a thick grey-white membrane (a pseudomembrane) that forms over the tonsils, throat, or nose and can obstruct breathing. The name derives from the Greek word for "leather," describing this tough membrane, and the physician Hippocrates left the earliest known descriptions of throat conditions consistent with the disease.

Diphtheria has a long documented history. The German bacteriologist Edwin Klebs identified the causative bacterium in 1883, and Friedrich Löffler cultivated it and demonstrated its role in the disease the following year — which is why Corynebacterium diphtheriae is sometimes called the Klebs-Löffler bacillus. Before the introduction of antitoxin and vaccines, diphtheria was one of the leading causes of childhood death worldwide.

Who is most likely to get diphtheria?

Unvaccinated or under-vaccinated people are by far the most susceptible to diphtheria, with young children historically at greatest risk. Susceptibility falls after the age of five, and after 15 the disease appears comparatively rarely. The younger the child, the more dangerous diphtheria tends to be. Additional risk groups include:

• Children who have not completed their primary vaccination course.
• Adults whose childhood immunity has waned without booster doses.
• Travellers to and residents of regions where the disease still circulates, such as parts of South Asia, Southeast Asia, and Sub-Saharan Africa.
• People living in crowded or unsanitary conditions, or in areas where health infrastructure has been disrupted by conflict or crisis.

The causative agent — the diphtheria bacillus

The microbes that cause diphtheria are called diphtheria bacilli, known scientifically as Corynebacterium diphtheriae. Like the microbes of scarlet fever, they live mainly in the nose, nasopharynx, throat, and mouth of an infected person. For that reason diphtheria spreads in broadly the same way as scarlet fever. Corynebacterium diphtheriae is a Gram-positive, non-motile, club-shaped rod that often arranges in characteristic V- or L-shaped clusters under the microscope.

Bacterial strains and their classification

Only strains of Corynebacterium diphtheriae that produce diphtheria toxin cause the severe form of the disease, and toxin production depends on infection of the bacterium by a bacteriophage. The bacterium is traditionally divided into four biotypes — gravis, mitis, intermedius, and belfanti — based on colony appearance and biochemical properties. Toxigenicity is not tied to a single biotype: it is conferred by a virus called a corynebacteriophage, which carries the tox gene. When this phage infects the bacterium (a process known as lysogenic or phage conversion), the bacterium gains the ability to synthesize the toxin. A related organism, Corynebacterium ulcerans, can also carry the toxin gene and cause a diphtheria-like illness, often linked to contact with animals or unpasteurized dairy.

How diphtheria toxin works

Diphtheria toxin causes disease by shutting down protein synthesis inside human cells. The toxin has two parts: one fragment binds to the cell surface and delivers the second fragment into the cell, where it chemically modifies (ADP-ribosylates) a molecule called elongation factor 2. Because elongation factor 2 is essential for assembling proteins, its inactivation kills the cell. Locally this cell death produces the fibrin-rich pseudomembrane; when the toxin enters the bloodstream it can damage distant organs, most dangerously the heart muscle and peripheral nerves.

How diphtheria is transmitted

Diphtheria spreads mainly from person to person through respiratory droplets, but it can also be caught in other ways. Infection most often happens during close contact with a sick person who coughs, sneezes, or talks, but you can also become infected through food, and through objects the patient used or that were near them — care items, dishes, toys, linens, books, and so on.

Infection through food and objects

Contaminated food and shared items can transmit diphtheria because the bacteria survive for a time outside the body on surfaces and in droplets deposited on them. Unpasteurized milk has been implicated in some outbreaks, and everyday possessions handled by a patient — utensils, cups, toys, and bedding — can carry infectious material to another person.

Mechanisms of infection transmission

The principal routes by which diphtheria passes between people are:

• Airborne respiratory droplets — the most common route, released when an infected person coughs, sneezes, or speaks.
• Direct contact with secretions from the nose, throat, eyes, or skin lesions of a patient.
• Indirect contact through contaminated objects and, occasionally, food.
• Contact with skin lesions in cutaneous diphtheria, which is an efficient source of transmission in warm climates and crowded settings.

Healthy people can also carry the bacterium in the throat without symptoms and unknowingly pass it on, which is why identifying and treating carriers is an important part of outbreak control.

The contagious period of the disease

An untreated person with diphtheria is generally contagious for about two to four weeks, and occasionally longer if a chronic carrier state develops. With appropriate antibiotic treatment the contagious period is shortened dramatically — usually to within 48 hours of starting therapy. Public health authorities confirm that a patient is no longer infectious by taking two negative cultures collected at least 24 hours apart, after antibiotics have finished.

The course of diphtheria

Diphtheria usually begins acutely, though it sometimes develops gradually. On the first day the child becomes listless, sluggish, and irritable; the temperature rises to roughly 37.5–38 °C, and sometimes higher. On examining the throat, redness of the mucous membrane of the palate and the back of the throat is found. The tonsils are also reddened and enlarged and become coated with a membrane. In the patient the cervical lymph nodes swell markedly and become painful to the touch.

The latent (incubation) period

The incubation period of diphtheria lasts from 2 to 7 days, and sometimes longer. During this quiet interval the person shows no symptoms but may already be able to infect others, which is one reason the disease can spread before it is recognized. Because the early signs resemble an ordinary sore throat, prompt medical assessment is important whenever diphtheria is possible.

Signs and symptoms of diphtheria

The signs of diphtheria depend on where the bacteria take hold, but common early symptoms include a sore throat, low-grade fever, weakness, and swollen "bull-neck" lymph nodes. The defining feature is the greyish pseudomembrane that adheres firmly to the affected tissue and bleeds if scraped. As the toxin spreads, more serious symptoms — difficulty breathing or swallowing, changes in vision, slurred speech, and signs of heart strain — can appear.

Clinical picture according to location

Diphtheria is classified by the anatomic site it affects, and the presentation differs accordingly. The two broad categories are respiratory diphtheria (nose, throat, tonsils, and larynx) and non-respiratory diphtheria, which most often means the skin.

Diphtheria of the throat and pharynx

Pharyngeal and tonsillar diphtheria is the most common form and produces the classic sore throat, difficulty swallowing, fever, and an adherent membrane over the tonsils and throat wall. When the membrane extends into the larynx it causes diphtheritic croup, with a barking cough, hoarseness, and progressive airway obstruction that can become a life-threatening emergency. Care for a child with an inflamed throat must be organized much as it is in a case of tonsillitis (angina), but with urgent medical supervision because the airway risk is real.

Cutaneous diphtheria

Cutaneous diphtheria affects the skin rather than the respiratory tract, producing chronic, non-healing ulcers often covered by a grey membrane. It tends to be milder in terms of toxin-related complications but is highly contagious and acts as a reservoir for spreading the bacterium, especially in tropical regions and among people living in crowded or unsanitary conditions.

Diagnosis of diphtheria

Diphtheria is diagnosed by combining the clinical picture — particularly the adherent pseudomembrane — with laboratory confirmation. Because treatment must not wait for laboratory results, doctors start antitoxin and antibiotics on clinical suspicion while samples are processed. A case is classified as confirmed, probable, or suspected depending on how the clinical, laboratory, and epidemiological evidence line up, following criteria used by public health agencies such as the Centers for Disease Control and Prevention (CDC).

Culture methods and identification of the pathogen

Laboratory confirmation of diphtheria relies on culturing Corynebacterium diphtheriae from swabs of the throat, nose, or skin lesion. The steps typically include:

• Collecting a swab from beneath the membrane, where the bacteria are most concentrated.
• Growing the sample on selective media such as Löffler's or tellurite-containing agar.
• Identifying the organism by its colony appearance and biochemical reactions.
• Testing whether the isolate produces toxin — historically with the Elek immunodiffusion test and today often with PCR to detect the tox gene.

Complications of diphtheria

The complications of diphtheria arise mainly from the toxin spreading through the body and from mechanical airway obstruction. The most dangerous outcomes are suffocation from an expanding membrane, inflammation of the heart, and damage to the nervous system. The risk of complications rises the longer treatment is delayed.

Heart damage (myocarditis)

Myocarditis — inflammation of the heart muscle caused by the diphtheria toxin — is one of the leading causes of death in diphtheria. It can appear within the first week or up to several weeks into the illness, producing abnormal heart rhythms, a weakened heartbeat, and, in severe cases, heart failure. Patients with signs of cardiac involvement need close monitoring in hospital.

Damage to the nervous system (neuritis)

Diphtheria toxin can also injure peripheral nerves, a condition called neuritis or neuropathy. It often begins with paralysis of the soft palate and difficulty swallowing, and may progress to blurred vision, slurred speech, and weakness of the limbs or the muscles of breathing. These neurological effects usually appear later in the course of the disease and can be slow to resolve, though most survivors recover.

Treatment of diphtheria

Diphtheria is treated with two things at once — diphtheria antitoxin to neutralize the poison and antibiotics to kill the bacteria — together with supportive hospital care. Because the antitoxin only works on toxin that has not yet entered cells, treatment is started immediately on clinical suspicion, without waiting for laboratory confirmation. Patients are isolated to prevent spread, and the airway is protected whenever the membrane threatens breathing.

Use of antitoxin

Diphtheria antitoxin is the cornerstone of treatment: it binds and neutralizes circulating toxin before it can attach to and damage the heart and nerves. It is given as early as possible, since any delay allows more toxin to enter cells beyond its reach. Because the antitoxin is produced in horses, patients are tested for hypersensitivity before administration and monitored for allergic reactions.

Antibiotic therapy

Antibiotics are given to eliminate Corynebacterium diphtheriae, stop toxin production, and shorten the period of contagiousness. The mainstays are:

• Penicillin — given by injection or, later, orally.
• Erythromycin — an alternative, particularly for people allergic to penicillin.

A typical course runs about 14 days, after which cultures are repeated to confirm the bacterium has been cleared. Antibiotics complement, but do not replace, antitoxin therapy.

Resistance of the pathogen to antimicrobial drugs

Antimicrobial resistance in Corynebacterium diphtheriae is an emerging concern, with reports of strains showing reduced susceptibility to penicillin and erythromycin. Because resistance can undermine standard therapy, laboratories increasingly perform antimicrobial susceptibility testing on isolates, and treatment may need to be adjusted based on the results. Ongoing surveillance of resistant strains helps guide public health recommendations.

Caring for a sick child

If a child has fallen ill with diphtheria, the priority is to get medical help immediately and to avoid spreading the infection to others. Call a doctor to the sick child's home right away rather than taking a child with a sore throat to a clinic or outpatient department, because there the child could infect other children — and moving the patient may also worsen their condition, since diphtheria is considered dangerous. Care for the child should be organized as in a case of tonsillitis, but always under a doctor's direction.

What to do if a child falls ill

If a child develops diphtheria, parents should notify the kindergarten or school the child attended before the illness so that preventive measures can be taken for the other children who had previously been in contact with them. If there are other children in the family or apartment, they too must not be allowed into the nursery, kindergarten, or school until a doctor has examined them. The essential steps are:

• Call a doctor to the home without delay and keep the sick child isolated.
• Do not transport a child with a sore throat to a clinic where they could infect others.
• Inform the school or kindergarten so contacts can be assessed.
• Keep other children in the household away from group settings until they are examined.
• Watch for warning signs — difficulty breathing or swallowing, a muffled voice, or a bluish tinge to the skin — and seek emergency care if they appear.

Prevention of diphtheria

Diphtheria is prevented most effectively through vaccination, backed up by rapid identification and treatment of contacts around any case. Widespread immunization is the reason the disease has become rare in countries such as the United States, and the World Health Organization, together with UNICEF, promotes diphtheria vaccination as part of routine childhood immunization worldwide.

Vaccination: combination vaccines

Diphtheria vaccines contain diphtheria toxoid — an inactivated form of the toxin that trains the immune system without causing disease — and are almost always combined with protection against tetanus and pertussis. The choice of vaccine depends on age:

• DTaP — for children under seven, covering diphtheria, tetanus, and pertussis.
• DT — diphtheria and tetanus for young children who cannot receive the pertussis component.
• Tdap — for adolescents and adults, with a reduced diphtheria dose.
• Td — tetanus and diphtheria boosters for older children and adults.
• Pentavalent vaccine — used in many countries to protect against five diseases in a single shot.

In the United States, children typically receive a five-dose DTaP series in infancy and early childhood, and several states, including New York State, require diphtheria-containing vaccination for school entry.

Revaccination and booster dose recommendations

Immunity from diphtheria vaccination and even from natural infection fades over time, so booster doses are needed to keep protection strong. The CDC recommends a Tdap dose in adolescence, followed by a Td or Tdap booster every ten years throughout adult life. Pregnant women are advised to receive Tdap during each pregnancy to protect both mother and newborn.

Identifying contacts and preventive treatment

Around every case of diphtheria, public health workers trace close contacts and give them preventive treatment to stop the chain of transmission. Diphtheria is a notifiable disease, meaning cases must be reported promptly to the local health department. Standard measures for contacts include:

• Taking throat and nose cultures to detect carriers.
• Giving a course of antibiotics (penicillin or erythromycin) as prophylaxis.
• Checking vaccination status and administering a booster if needed.
• Monitoring contacts for symptoms during the incubation period.

Mortality and prognosis in diphtheria

Even with modern treatment, respiratory diphtheria carries a case-fatality rate of roughly 5 to 10 percent, and higher in young children or when treatment is delayed. Historically, before antitoxin and vaccines, the disease killed a large share of the children it infected. The outlook is best when antitoxin is given early, before the toxin has damaged the heart and nerves, which is why speed of diagnosis and treatment matters so much. Globally the disease remains a threat in areas with low vaccination coverage, and disruptions to immunization services — such as those seen during the COVID-19 pandemic — can allow outbreaks to re-emerge. To learn more about related health topics, see our medicine section.