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