Coryneform bacteria are normal residents of the skin. They can also cause opportunistic infections. These are infections that occur as a secondary infection, when the immune response of a host has been weakened by another infection or by another insult to the immune system, such as chemotherapy. An example is Corynebacterium jeikeium, whose infection can be taxing to treat as the organism is resistant to numerous antibiotics. Over the past two decades, the numbers of such infections have been rising. This may be an indication of an immune stress on the body.

Coryneform bacteria can stain positive in the Gram stain protocol. However, this reaction is not consistent. A characteristic feature is their tendency to arrange themselves in a V-like pattern or lined up, much like logs stacked one against the other.

While there are some consistencies among the members of the Coryneform bacteria, a hallmark of these bacteria is their diversity of habitats. This, and their inconsistent Gram stain reaction, can make identification of the microorganism tedious. More rigorous biochemical and molecular biological tools of identification are being used by organizations such as the Centers for Disease Control (CDC) to establish a definitive classification scheme for Coryneform bacteria. For example, CDC groups JK and D-2 in the genus Corynebacterium are now recognized as important human disease-causing microorganisms. Conversely, these rigorous techniques have resulted in the removal of some species of bacteria from the genus.

Coryneform bacteria are important medically. Corynebacterium diphtheriae is the organism that causes diphtheria. In fact, before Coryneform bacteria were known to be comprised of several species, the bacteria were referred to as diphtheroids. Diphtheria is apparent as an inflammation and bleeding of the throat and as a generalized toxic poisoning of the body, due to the release of a powerful toxin by the bacteria. The toxin spreads throughout the body via the bloodstream and has a particular affinity for tissues such as the heart, nerve endings and the adrenal glands. Diphtheria is treatable with antibiotics.

Other species of the genus Corynebacterium cause mastitis in cows (an infection and inflammation of the udder), infection of the lymph nodes of sheep, and skin rashes and ulcerations in humans. Rhodococcus equi, which inhabits soil, is an important pathogen of young horses. Another human pathogen is It is also a normal resident on skin surfaces, and can cause an infection in those receiving chemotherapy.