Influenza virus particles, computer artwork. The particles comprise a core (central sphere) surrounded by membranes. The outer membrane bears the proteins neuraminidase and haemagglutinin (spikes), which help the virus particle enter a host cell. The virus uses its host cell's genetic machinery to build more copies of itself. These copies then make a new outer membrane from the cell's own membrane, and erupt from the cell to infect others. Influenza virus is a type of Orthomyxovirus. It causes fever, weakness and aching muscles.