Respiratory syncytial virus (RSV) particles, coloured transmission electron micrograph (TEM). This pneumovirus, a type of paramyxovirus, is a major cause of human respiratory tract infections in patients of all ages. Each virion consists of RNA (ribonucleic acid) genetic material enclosed in a protein coat, or capsid (orange), within a phospholipid envelope (cream). The envelope is covered in protein spikes (green), which enable the virus to attach to and enter a host cell. In adults, RSV only affects the upper respiratory tract, but in infants bronchiolitis (bronchiole inflammation) or bronchopneumonia can result. Magnification: x118,000 when printed 10 centimetres wide.