Influenza viruses in cell. Coloured scanning electron micrograph (SEM) of influenza viruses (tiny yellow spheres), the cause of human influenza, infecting a cell. The cell's cytoplasm (green) and nucleus (light red, at upper right) are covered in the viruses and some more are seen outside the cell. Like all viruses, influenza (flu) viruses take control of the cell, turning the nucleus into a factory for the production of more viruses. These viruses belong to the orthomyxovirus class of RNA (ribonucleic acid) viruses, all of which have an affinity for mucus. Flu is usually short-lived and seldom serious. Magnification: x3,400 at 6x7cm size. Magnificatio: x11,600 at 7.5x9.5inch size.