Attenuated viruses and methods of designing them are disclosed. In one embodiment, there is disclosed an attenuated form of a virulent virus comprising an RNA encoding a viral protein or a nucleic acid sequence transcribable to said RNA, wherein the folding energy or structure of the RNA is changed at positions of evolutionarily conserved RNA structures with respect to that of said RNA encoding said viral protein in the virulent virus so as to bring about attenuation of the virus.