Aspergillus is widespread in nature and produces small conidia that are easily aerosolized. Atopic individuals often develop severe allergic reactions to the conidial antigens. In immunocompromised patients, the conidia may germinate to produce hyphae that invade the lungs and other tissues. Progress of disease can be rapid. A diagnosis of aspergillosis is supported by a tissue biopsy showing invasion by the organism and a positive culture from a normally sterile site. Aspergilli may be airborne in the environment and be laboratory culture contaminants or present in orally obtained samples from patients without apparent clinical illness and at low risk for invasive aspergillosis and such finding should be interpreted with caution. In tissue Aspergillus spp. (most commonly A. fumigatus) have septate hyphae 3 to 6 mm in width that are described as having acute angle branching. In bone marrow transplant patient's infection may occur early after transplant or after several months as in this case. The other infections can occur in a compromised host but would differ from what is shown. Particularly, the septate hyphae and acute angle branching are not consistent with agents of mucormycosis that have wide non-septate hyphae. Histoplasma capsulatum, which causes histoplasmosis in humans, grows in yeast form in the infected person. Normal healthy individuals may be infected; however, in the immunocompromised host the infection can be more severe.