Bluetongue virus capsid, molecular model. This virus, transmitted by midges, causes bluetongue disease in ruminants (commonly sheep, and less frequently cattle, goats and deer). 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.