Rift Valley fever virus. Coloured transmission electron micrograph (TEM) of three Rift Valley fever virus particles (virions). They each possess an envelope (yellow) derived from the host cell membrane. This is studded with proteins (fringe, green), which may be used to identify the virus. Rift Valley fever occurs in Africa, infecting humans, cattle, sheep, rodents and other species. It is transmitted by insects, mainly mosquitoes, and causes chills, fever, a headache and joint and muscle pain. It is rarely fatal, but can cause retinal haemorrhages that may lead to visual impairment. Magnification: x51,900 at 6x7cm size.