Influenza A virus (RNA virus, Orthomyxoviridae Family), animated coloured transmission electron micrograph (TEM). 'Influenza Virus', often know as 'The Flu Virus' often changes or mutates, complicating the development of suitable vaccines for all strains. As a result, epidemics or even occasional pandemics of influenza occur. The flu virus enters the body when small respiratory secretions from an infected person are inhaled. Influenza A virions have surface projections that contain the receptors by which the virus attaches to the host's respiratory tract epithelial cells. Symptoms commonly include headache, fever, muscle pain, cough, and generalized weakness. The young, elderly, and those with a weakened immune system are most affected. Original image courtesy of NIH. Magnification: x23, 785 when shortest axis printed at 25 millimetres.