Adenovirus capsid model, computer artwork. Adenoviruses are the largest viruses not to have a protein coat, as they are small enough to enter the cell membrane without binding to it first. They only have a protein capsid enclosing their DNA genetic material. This capsid is icosahedral, having 20 triangular faces (yellow/orange), and made up of regular arrangements of protein capsomers (gold/green). There are many types of adenovirus. They commonly cause infections of the upper respiratory tract, causing cold-like symptoms, particularly in children.