Bluetongue virus (BTV) capsid, molecular model. BTV is an orbivirus, and is of major economic importance as an insect-borne pathogen of cattle and other ruminants. It is named after the cyanotic (blue) tongue it causes in sheep. It is not transmitted to humans. In viruses, the capsid is the protein shell that encloses the genetic material. It 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 capsid functions is to aid transmission of the viral genetic material into host cells.