Avian flu virus. Computer artwork of an avian influenza (bird flu) virus. Each virus consists of a core of RNA (ribonucleic acid, blue and purple) genetic material surrounded by a protein coat. Embedded in the coat are the surface proteins (spikes and bubbles) haemagglutinin (H) and neuraminidase (N), each of which exist in several subtypes. Haemagglutinin binds to host cells, allowing the virus to enter them and replicate. Neuraminidase allows the new particles to exit the host after replication. The virus that is currently endemic in birds is H5N1. Humans that have contact with infected birds can become infected, but as of January 2007 no human to human transmission has been recorded.