Dengue fever virus particles. Coloured transmission electron micrograph (TEM) of dengue fever virus particles (yellow) in a cell. These virions are covering themselves with an envelope (light blue) derived from the host cell membrane. After enveloping they will burst out of the cell. Dengue fever is transmitted by mosquitoes and the virus replicates chiefly in the immune system's macrophage cells. It causes fever, a headache and joint pain, often with a rash. In severe cases the victim can suffer haemorrhages and hypovolaemia, a dangerously low level of circulating blood. Magnification: x80,400 at 6x7cm size.