Adenoviruses are medium-sized (90–100 nm), nonenveloped (without an outer lipid bilayer) icosahedral viruses composed of a nucleocapsid and a double-stranded linear DNA genome. There are 55 described serotypes in humans, which are responsible for 5–10% of upper respiratory infections in children, and many infections in adults as well. Viruses of the family Adenoviridae infect various species of vertebrates, including humans. There are 56 accepted human adenovirus types (HAdV-1 to 56) in seven species (Human adenovirus A to G).