Norwalk virus capsid, molecular model. This norovirus, which causes a viral form of gastroenteritis, is transmitted from person-to-person or through contaminated food. In viruses, the capsid is the protein shell that encloses the genetic material. A capsid consists of subunits called capsomeres that self-assemble to form the shell seen here. One of the functions of the capsid is to aid the transmission of the viral genetic material into host cells.