Rationale: Idiopathic pulmonary fibrosis is a progressive disease with high mortality. While most patients have a slow, progressive course, some patients will have an acute deterioration in function or acute exacerbation which carries a poor prognosis. In some cases, acute deterioration is associated with infection. Herpesviruses have been associated with this disease. Fibrocytes have also been shown to be important in the pathogenesis of pulmonary fibrosis. Objectives: To develop a murine model for infectious exacerbation of pre-existing fibrosis, and provide mechanistic insight into the role of herpesviruses in fibrotic disease. Methods: We used a model of fluorescein isothiocyanate-induced pulmonary fibrosis in mice. Infection with a murine gammaherpesvirus was given at time of established lung fibrosis. Measurements were made at the time of peak lytic viral replication. Measurements and Main Results: We demonstrate that infection with gammaherpesvirus can exacerbate established fluorescein isothiocyanate-induced fibrosis evidenced by increased total lung collagen, histologic changes of acute lung injury and diminished lung function. Gammaherpesvirus can exacerbate pre-existing fibrosis in a Th1 cytokine environment and in the absence of Th2 cytokines. Gammaherpesvirus increases fibrocyte recruitment to the lung in wild-type, but not CCR2-/- mice, in part because viral infection upregulates production of CCL2 and CCL12, chemokines important for fibrocyte recruitment. In contrast, mouse adenovirus infection did not exacerbate collagen deposition. Conclusions: These data provide a new model for gammaherpesvirus exacerbation of established pulmonary fibrosis. The upregulation of chemokines during viral infection and subsequent recruitment of fibrocytes to the lung likely contribute to augmentation of pulmonary fibrosis.