Current advancement in nanotechnology provides a novel platform for the development of potential and effective agents by modifying the materials at nanolevel with remarkable physicochemical properties, high surface area to volume ratio and increased reactivity. Among metal nanoparticles, silver nanoparticles have strong antibacterial, antifungal and antiviral potential to boost the host immunity against pathogen attack. Since metals may attack a broad range of targets in the virus there is a lower possibility to develop resistance as compared to conventional antivirals. Nevertheless, the interaction of silver nanoparticles with viruses is a largely unexplored field. We have analysed silver nanoparticles produced from different fungi showing that their antiviral activity is dependent on the production system used. We demonstrated that silver nanoparticles undergo a size-dependent interaction with HSV-1 and HSV-2 and with human parainfluenza virus type 3; the nanoparticles are capable of reducing viral infectivity, probably by blocking interaction of the virus with the cell, which might depend on the size and zeta potential of the silver nanoparticles.