Influenza virus. Computer artwork of the structure of an influenza ('flu) virus. Influenza viruses belong to the orthomyxovirus class of RNA viruses (the influenza viruses group), which possess an affinity for mucous. The coil of ribonucleic acid (RNA) is seen as a white coil in the centre of the purple outer casing, or capsid. Influenza causes fever, headaches, fatigue and muscle aching. The infection is not generally serious, but certain rare strains can lead to pneumonia and death. There are a number of anti-viral drugs available, but for the majority of cases, bed rest, warm drinks and painkillers will suffice.