The four subspecies differ slightly in their colonial morphology and biochemical properties, such as the ability to metabolize certain nutrients.

However, when it comes to the subtyping of C. diphtheriae, there is not a lot of useful or accurate classification due to the lack of publicly available resources to identify strains and therefore find the origin of outbreaks.

[11][12] The diphtheria toxin gene is encoded by the bacteriophage found in toxigenic strains, integrated into the bacterial chromosome.

[15] To identify C. diphtheriae, a Gram stain is performed to show Gram-positive, highly pleomorphic organisms often looking like Chinese letters.

Immunocompromised individuals, poorly immunized adults, and unvaccinated children are at the greatest risk of contracting diphtheria.

A thick, grey coating accumulates in the nasopharyngeal region, making breathing and swallowing more difficult.

[20] If left untreated, diphtheria toxin may enter the bloodstream, causing damage to the kidneys, nerves, and heart.

A vaccine, DTaP, effectively prevents the disease and is mandatory in the United States for participation in public education and some professions (exceptions apply).

[21] The bacteria have several virulence factors to help them localize on areas of the respiratory tract, many of which are yet to be fully understood as diphtheria does not affect many model hosts such as mice.

One common virulence factor that has been studied in vitro is DIP0733, a multi-functional protein that has been shown to have a role in bacterial adhesion to host cells and fibrogen-binding qualities.

The ability to bind to extracellular matrices aids the bacteria in avoiding detection by the body's immune system.

Individuals with faucial diphtheria typically have the pseudomembrane grow over the tonsil and accessory structures, uvula, soft palate, and possibly the nasopharyngeal area.

[21] Mode of transmission is person-to-person contact via respiratory droplets (i.e., coughing or sneezing), and less commonly, by touching open sores or contaminated surfaces.

[10] A vaccine, DTaP, effectively prevents the disease and is mandatory in the United States for participation in public education and some professions (exceptions apply).

The invention of the toxoid vaccine, which provides protection against Corynebacterium diphtheriae, caused a dramatic shift in the bacterium's rate of infection in the United States.

According to the National Health and Nutrition Examination Survey (NHANES), "80 per cent of persons age 12 to 19 years were immune to diphtheria" due to the wide use of the vaccine in the United States.

[24] When a toxigenic strain of Corynebacterium diphtheriae infects the human body, it releases harmful toxins, especially to the throat.

[25] The wide use of the diphtheria vaccine dramatically decreased the rate of infection and allows for primary prevention of the disease.

[8] In the United States, the DTaP vaccine to parents of infants which typically involves a series of five shots is recommended.

[8] Possible side events that are associated with the diphtheria vaccine include "mild fever, fussiness, drowsiness or tenderness at the injection site".