Eosinophils are implicated in the pathophysiology of respiratory virus infection, most typically in negative roles, such as promoting wheezing and bronchoconstriction in conjunction with virus-induced exacerbations of reactive airways disease and in association with aberrant hypersensitivity responses to viral vaccines. However, experiments carried out in vitro and in vivo suggest positive roles for eosinophils, as they have been shown to reduce virus infectivity in tissue culture and promote clearance of the human pathogen, respiratory syncytial virus in a mouse challenge model. The related natural rodent pathogen, pneumonia virus of mice (PVM), is highly virulent in mice, and is not readily cleared by eosinophils in vivo. Interestingly, PVM replicates in eosinophils and promotes cytokine release. The molecular basis of virus infection in eosinophils and its relationship to disease outcome is currently under study.