Influenza virus (an Orthomyxovirus) is responsible for acute upper respiratory disease, usually accompanied by fever and myalgia.
Virions are usually roughly spherical and about 200nm in diameter.
The envelope contains rigid "spikes" of haemagglutinin and neuraminidase which form a characteristic halo of projections around negatively stained virus particles.
The viral genome is composed of eight segments of ssRNA.
The helical ribonucleo-protein is not often seen,
but occasional particles show evidence of internal helical components.