Illustration of the respiratory syncytial virus (RSV, pale blue), also known as human orthopneumovirus. RSV, also known as human orthopneumovirus, causes infections in the lungs and respiratory tract, which is very common in children, but it can also affect adults. The symptoms are usually mild and resemble those of the common cold. The initial stages of infection are controlled by the two major glycoproteins present on the surface of the RSV virion: the attachment glycoprotein (grey) and the fusion glycoprotein (darker blue). The fusion protein is the main target for vaccine development and the major antigenic site is highlighted in red.