Hypersensitivity pneumonitis (HP) is a worldwide lung disease caused by an immune response to a variety of mostly organic inhaled antigens. Only a small percentage (for example, 3 in 1000 dairy farmers) of subjects exposed to HP antigens develop the disease. It is possible that cofactors are needed to render the patient hypersensitive to environmental antigens; recent studies suggest that a viral infection could be such a trigger factor. Mice infected once with Sendai virus and simultaneously sensitized with HP antigens develop an enhanced response to the antigen that persists long after the transient viral infection has waned. Moreover, viral antigens were found in the lavage cells and lung tissue of patients with HP. Viral infections are known modulators of the immune response; viruses increase the antigen-presenting capacity of alveolar macrophages, decrease phagocytosis and antigen clearance, induce the release of pro-inflammatory cytokines, and favor the proliferation of Th1 T-lymphocytes (cells putatively associated with HP).