Animation showing the structure of an adenovirus as it is built up component by component. At its core is a double-stranded DNA genome (red). This is enclosed in an icosahedral capsid made up of 12 penton bases (yellow) at the vertices, each surrounded by five peripentonal proteins (light blue). The rest of the capsid is made up of 240 hexon proteins (dark blue). Each penton base carries a fibre protein, whose tip allows it to bind to and enter host cells. Adenoviruses typically cause upper respiratory tract infections. Their large capsids have made them popular as carriers of genes in gene therapy or for vaccines, such as some of the covid-19 vaccines.