Aspergillus fumigatus is one of the most ubiquitous of the airborne saproph
ytic fungi. Humans and animals constantly inhale numerous conidia of this f
ungus. The conidia are normally eliminated in the immunocompetent host by i
nnate immune mechanisms, and aspergilloma and allergic bronchopulmonary asp
ergillosis, uncommon clinical syndromes, are the only infections observed i
n such hosts. Thus, A. fumigatus was considered for years to be a weak path
ogen. With increases in the number of immunosuppressed patients, however; t
here has been a dramatic increase in severe and usually fatal invasive aspe
rgillosis, now the most common mold infection worldwide. In this review, th
e focus is on the biology of A. fumigatus and the diseases it causes. Inclu
ded are discussions of (i) genomic and molecular characterization of the or
ganism, (ii) clinical and laboratory methods available for the diagnosis of
aspergillosis in immunocompetent and immunocompromised hosts, (iii) identi
fication of host and fungal factors that play a role in the establishment o
f the fungus in vivo, and (iv) problems associated with antifungal therapy.