The recent SARS outbreak has spawned a major controversy concerning the protective performance of facemasks. The objective of this paper is to establish a mathematical model for describing the mechanism of viruses diffusion in a facemask during breathing cycles. In this model the mechanisms of the water vapor molecular diffusion, evaporation/ condensation, sorption/desorption of moisture by fibers, Brownian diffusion of the virus, filtration of virus, capillary penetration of the viruses with the liquid water and latent heat generation due to phase change are considered. Meanwhile, the theoretical predictions are compared with experimental data, and good agreement is observed between the two, indicating that the model is satisfactory. Also, the effects of structural and materials properties on virus transfer are investigated. (c) 2005 Elsevier Ltd. All rights reserved.