Influenza viruses, coloured transmission electron micrograph (TEM). This section through several influenza (flu) virus particles shows the central core of the particles (orange), which contains the genetic material. The outer coat (green) consists of proteins by which the virus particles attach to a host cell. Influenza is in the orthomyxovirus group of viruses. It infects the respiratory tract and causes fever, aching muscles, a sore throat and feelings of fatigue. New strains of the virus develop rapidly, and can cause epidemics.