Background. Protein energy malnutrition (PEM), a common cause of secondary immune deficiency in children, is associated with an increased risk of infections. Very few studies have addressed the relevance of PEM as a risk factor for influenza.Methods. We investigated the influence of PEM on susceptibility to, and immune responses following, influenza virus infection using isocaloric diets providing either adequate protein (AP; 18%) or very low protein (VLP; 2%) in a mouse model.Results. We found that mice maintained on the VLP diet, when compared to mice fed with the AP diet, exhibited more severe disease following influenza infection based on virus persistence, trafficking of inflammatory cell types to the lung tissue, and virus-induced mortality. Furthermore, groups of mice maintained on the VLP diet showed significantly lower virus-specific antibody response and a reduction in influenza nuclear protein-specific CD8+ T cells compared with mice fed on the AP diet. Importantly, switching diets for the group maintained on the VLP diet to the AP diet improved virus clearance, as well as protective immunity to viral challenge.Conclusions. Our results highlight the impact of protein energy on immunity to influenza infection and suggest that balanced protein energy replenishment may be one strategy to boost immunity against influenza viral infections. © 2012 The Author.