Nocardia is found worldwide in soils and dust.   In immunocompromised patients the lungs appear to the most common initial site of infection by the inhalation of free living organisms.   Pulmonary nocardiosis  may mimic tuberculosis, staphylococcal, or mycotic infections.  Disseminated disease mainly occurs in immunocompromised patients with underlying illnesses such as chronic granulomatous disease, human immunodeficiency virus (HIV) infection,  in patients undergoing cytotoxic chemotherapy, organ transplantation, or prolonged glucocorticoid treatment.  Subcutaneous infection occurs  from trauma related inoculation of the organism.  Nocardia asteroides is the most commonly isolated pathogenic Nocardia species.

In direct gram smears they appear as very long, branching, filamentous beading gram positive bacilli and are indistinguishable from Actinomyces spp. However, in contrast to Actinomyces spp., when stained with a modified Kinyuon stain they appear partially acid fast (blue and pink).   Colonial morphology varies but most clinical isolates appear “chaulk” white or off white, dry, powdery with aerial hyphae.  Often colonies can be picked off the agar surface in entirety.