It forms partially acid-fast beaded branching filaments (appearing similar to fungi, but being truly bacteria).

[4] Most Nocardia infections are acquired by inhalation of the bacteria or through traumatic introduction through openings in epithelial barriers.

The various species of Nocardia are pathogenic bacteria with low virulence; therefore clinically significant disease most frequently occurs as an opportunistic infection in those with a weak immune system, such as small children, the elderly, and the immunocompromised (most typically, HIV).

Nocardial virulence factors are the enzymes catalase and superoxide dismutase (which inactivate reactive oxygen species that would otherwise prove toxic to the bacteria), as well as a "cord factor" (which interferes with phagocytosis by macrophages by preventing the fusion of the phagosome with the lysosome).

[citation needed] An important virulence factor in many pathogenic strains of Nocardia is trehalose dimycolate, which is also found in the bacterial genus Mycobacterium.

Nocardia may also cause a variety of cutaneous infections such as actinomycetoma (especially N. brasiliensis), lymphocutaneous disease, cellulitis, and subcutaneous abscesses.

[6] Nocardia isolation from biological specimens can be performed using an agar medium enriched with yeast extract and activated charcoal (BCYE), the same used for Legionella species.

[13] The genera Nocardia and Rhodococcus have been found to be closely related, supported by two conserved signature indels consisting of a one-amino-acid deletion in the alpha subunit of acetyl coenzyme A carboxylase (ACC), and a three-amino-acid insertion in a conserved region of an ATP-binding protein that are specifically shared by species from these two genera.

In addition, 14 hypothetical conserved signature proteins have been identified which are unique to the genera Nocardia and Rhodococcus.

[14] Nocardia comprises the following species:[1] The genus was named for Edmond Nocard (1850-1903), a French 19th-century veterinarian and biologist.