Calf diphtheria is a necrotic laryngitis that occurs in young calves (5 weeks old to 24 months old) worldwide, caused by an infection of the laryngeal tissue with Fusobacterium necrophorum bacterium. The condition can be acute or chronic and occurs sporadically year round but is most prevalent in the fall and winter months. It is thought to occur secondary due to prior laryngeal trauma or upper respiratory viral infection, often infectious bovine rhinotracheitis (IBR) or bovine respiratory syncytial virus (BRSV).

F. necrophorum is a normal inhabitant of healthy cattle but is opportunistic and when it multiples it causes the development of lesions throughout the calf's larynx which causes them painful respiratory distress and associated signs which differ depending on the severity and stage of the disease.