GA bacteriophage capsid, molecular model. Bacteriophages are viruses that infect bacteria, with the capsid forming the bacteriophage 'head'. The GA bacteriophage (formally Enterobacteria phage GA) infects enterobacteria. 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.