Rotavirus particle. Computer artwork (left) and coloured transmission electron micrograph (TEM) of the geometric structure of a rotavirus particle's capsid shell. The capsid of a rotavirus has 5-fold symmetry (shown by 5-line star at left and right), and is icosahedral in shape. An icosahedron has 20 triangular sides; five of these meet at the apex at the centre of the star. The cylindrical units at left (which correspond to the green areas at right) are capsomers, the protein units that make up a capsid. Inside the capsid is the genetic core of the virus. Rotavirus viruses can cause severe diarrhoea in children, killing hundreds of thousands worldwide every year. A rotavirus particle is around 70 nanometres in diameter.