Nocardia spp. are soil-dwelling, ubiquitous, gram-positive, filamentous, branching bacteria. The most common manifestation of nocardiosis is the pulmonary form, which typically affects patients with structural lung disease who have a malignancy or who are immunosuppressed.

Primary cutaneous nocardiosis (PCN) occurs after trauma (often with soil contact) to the skin and/or skin appendages and most often affects healthy patients. Lesions may occur suddenly and rapidly progress, reactivate after several years of dormancy, or slowly expand over 10 years. Lymphocutaneous spread may also be seen. A third manifestation is mycetoma. Here, a localized infection, typically of the foot, causes sinus tract formation and draining granules. Underlying tissues may become secondarily involved. Rarely, PCN can disseminate to other organs, but this is usually in the setting of significant immunocompromise.

PCN affects males more frequently than females. Risk factors include soil or sand exposure, gardening, farming, superficial injury from domestic shrubbery, intra-articular steroid injections, insect bites, cat scratches, or trauma from motor vehicle accidents leading to abrasions (ie, "road rash").

PCN may comprise up to 5% of all cases of nocardiosis. Nocardia infections can be found worldwide, but mycetomas are found more commonly in subtropical and tropical climates. PCN is usually caused by Nocardia brasiliensis but may be caused by nearly any species of Nocardia.

Secondary cutaneous nocardiosis occurs in association with underlying systemic nocardiosis. Fistulae form from underlying infected tissues. Subcutaneous abscesses are another manifestation. In underlying pulmonary disease, lesions occur over the chest. They are more widespread in disseminated disease.