This colorized negative-stained transmission electron micrograph (TEM) depicts the ultrastructural details of a number of influenza virus particles, or virions. A member of the taxonomic family Orthomyxoviridae, the influenza virus is a single-stranded RNA organism. The flu is a contagious respiratory illness caused by influenza viruses. It can cause mild to severe illness, and at times can lead to death. The best way to prevent this illness is by getting a flu vaccination each fall.