Bacteriophages, viruses that infect bacteria, have been shown to preferentially bind to mucosal surfaces. This has been suggested to provide an extra level of immunity against bacterial infections. Researchers tested this idea using fish, phages (viruses) and a fish-infecting bacteria. Phages were confirmed to bind to the mucosal surface, staying there for days and give protection from subsequent bacterial infection.

Source: Bacteria-infecting viruses bind mucosal surface and protect from disease