Parainfluenza virus. Transmission electron micrograph of a section through a human parainflu- enza virus (HPV). HPV causes croup (laryngotracheo bronchitis), pneumonia, and other diseases of the respiratory system. It is spread by personal cont- act and droplets in breath, and almost everybody has been infected with it by the age of six. The protein shell (capsid) of the virus is white. The outermost membrane, the lipoprotein envelope, is studded with projections (peplomers) which appear as a surface fringe. Within this is the protein shell (capsid, white) of the virus. This encloses the dark, coiled nucleoprotein (genetic material). Magnification: x51,300 at 10x8 inch size.