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. Here, this shell, approximately spherical in shape, has icosahedral symmetry. One of the functions of the capsid is to aid the transmission of the viral genetic material into host cells. The cell mechanisms are then used to produce new virus particles.