Influenza, more commonly known as flu, is caused by a virus which enters the body of birds, humans and other mammals by attacking their airways. The flu virus is remarkably simple and uses two elegant “keys” to get past the body defenses to infect cells and organs and reprogram their DNA systems to replicate itself. The first of these is hemagglutinin and the second is neuraminidase. There are fifteen variants of the first and nine of the second; for ease of discussion, scientists number these H1-H15 and N1-N9.