Diphtheria spreads primarily through respiratory droplets when an infected person coughs or sneezes. You breathe in those droplets, and the bacteria colonize your throat or nose. Less commonly, you can pick it up through direct contact with infected skin sores or by touching contaminated surfaces. The disease is rare in countries with high vaccination rates, but it still circulates worldwide, particularly where immunization coverage is low.
Respiratory Droplet Transmission
The most common way to contract diphtheria is by inhaling small respiratory droplets containing the bacterium Corynebacterium diphtheriae. Someone nearby coughs, sneezes, or even talks, and the bacteria become airborne in tiny moisture particles. If you’re close enough to breathe those in, the bacteria can settle in your throat, tonsils, or nasal passages and begin multiplying.
An infected person remains contagious as long as the bacteria are present in their respiratory secretions. Without antibiotic treatment, that window is typically about two weeks but can stretch up to six weeks in some cases. Antibiotics shorten contagiousness dramatically, usually clearing the bacteria within one to two days of starting treatment.
Skin Contact and Contaminated Surfaces
Diphtheria also has a skin-based form called cutaneous diphtheria, which is more common in tropical climates. In this form, bacteria infect open wounds or skin lesions, creating sores that can spread the infection to others through direct touch. You can also become infected by handling items soiled with discharge from these skin lesions, things like shared towels or clothing.
The bacteria are surprisingly hardy outside the body. C. diphtheriae can survive on dry surfaces for anywhere from 7 days to 6 months, according to biosafety data from the Public Health Agency of Canada. This means contaminated objects (called fomites) are a real, if less common, transmission route. Shared household items, doorknobs, or medical equipment could theoretically carry the bacteria long after contact with an infected person.
A Related Strain From Animals
A closely related bacterium, Corynebacterium ulcerans, can cause a diphtheria-like illness and is considered zoonotic, meaning it passes between animals and humans. Unlike classic diphtheria, human-to-human transmission of C. ulcerans has never been documented. The CDC reported the first known U.S. cases of toxin-producing C. ulcerans infection in humans who had concurrent colonization in their household pets, identified in Utah and Colorado between 2022 and 2023. If you have close contact with animals carrying this strain, particularly cats or dogs, there’s a small risk of contracting a diphtheria-like infection.
What Happens After the Bacteria Enter Your Body
Once C. diphtheriae reaches your throat, the incubation period is two to five days, though it can range from one to ten days. During this time, the bacteria begin producing a powerful toxin, but only if the particular strain carries a specific viral gene. Not all strains of C. diphtheriae are toxin-producing. The ones that are have been infected by a virus (a bacteriophage) that inserts the toxin gene into the bacterium’s DNA.
The toxin works by shutting down protein production inside your cells. It latches onto a receptor on the cell surface, gets pulled inside through normal cellular recycling processes, and then chemically disables a molecule your cells need to build proteins. Because protein synthesis is essential for virtually every cell function, this causes widespread tissue damage. In the throat, the dying cells, bacteria, white blood cells, and debris accumulate into a tough, grayish membrane that can block the airway. This “pseudomembrane” is the hallmark of respiratory diphtheria.
The toxin doesn’t stay local. It can enter the bloodstream and damage the heart muscle, nervous system, and kidneys, which is why diphtheria can be fatal even after the throat infection itself is under control.
Who Is Most at Risk
The single biggest risk factor is being unvaccinated or under-vaccinated. Diphtheria is rare in industrialized nations precisely because of widespread childhood immunization, but it persists in parts of the world where vaccination coverage is inconsistent. Travelers to these regions face higher exposure risk, especially if their boosters aren’t current.
Crowded living conditions increase transmission because more people share the same air and surfaces. Poor sanitation and limited access to healthcare compound the problem by allowing infections to go untreated longer, extending the contagious window. People with weakened immune systems or chronic skin conditions may also be more vulnerable to cutaneous diphtheria.
How Vaccination Prevents Infection
The diphtheria vaccine doesn’t prevent you from encountering the bacteria. Instead, it trains your immune system to neutralize the toxin before it can cause damage. The vaccine contains an inactivated form of the toxin (a toxoid) that’s harmless but still recognizable to your immune system.
The current U.S. immunization schedule calls for five doses of DTaP during childhood: at 2 months, 4 months, 6 months, 15 to 18 months, and 4 to 6 years. A booster with the Tdap formulation is recommended at age 11 to 12. After that, booster doses every ten years maintain immunity through adulthood. Vaccine-induced protection does wane over time, which is why those boosters matter. Adults who skip them gradually lose their defense against the toxin, even if they completed the full childhood series.