Illustration of a rotavirus revealing its inner structure layer by layer. This virus is pathogenic to humans and can cause several diarrhoeal diseases. Its capsid is a hull of three icosahedral layers that look like a wheel in electron micrographs. Six different viral proteins form the virion: VP1, VP2, VP3, VP4, VP6 and VP7. The RNA (ribonucleic acid) is segmented and double-stranded.