Influenza A H1N1 virus particles —
Influenza A viruses can infect humans,pigs, birds and horses. The H1N1 strain caused the Swine flu outbreak of 2009. At the center of each virus is its genetic fingerprint (the ribonucleic acid, pink), surrounded by a protective protein shell (the nucleocapsid, yellow). The enclosing fatty envelope (green) contains two types of protein, haemagglutinin and neuraminidase (the 'H' and 'N' in the strain's codename), the levels of which determine the strain of virus.