Dengue fever virus particles. Coloured transmission electron micrograph (TEM) of dengue fever virus particles (blue) in a cell. These virions are hidden within a vacuole (purple) within the host cell. They are transmitted to humans by mosquitoes, usually Aedes aegypti, and replicate chiefly in the macrophage cells of the immune system. The virus causes fever, a headache and joint pains, often accompanied by a rash. In very severe cases the victim can suffer haemorrhages and hypovolaemia, a dangerously low level of blood in the circulation. Magnification: x32,700 at 6x7cm size.