A recombinant bifunctional fusion protein comprises a first component which is the antigenic ectodomain of measles virus hemagglutinin protein (MeaH) genetically modified not to bind to human cells; and a second component fused thereto which is capable of binding to the surface structure of genetically variable viruses such as HCV or HIV or other therapeutic targets. The second component binds to the target and the first component is recognised by anti-measles antibodies present in the majority of the population. The protein may be used therapeutically to treat HCV or HIV infection or against other therapeutic targets.