Normal common bacteria resides on the skin of the pinna.
A breach of the skin can allow the normal bacteria that resides on the skin of the pinna to infect the skin and the underlying cartilage. Trauma to the pinna and cartilage is the most common cause of pinna infection. If the infection involves the cartilage, it is called perichondritis. Sometimes the infection is severe enough to destroy the cartilage and cause an abscess (pus collection).
Infection of the external ear canal may also spread to involve the pinna.