H5N1 avian flu virus, computer artwork. This virus became epizootic (epidemic, but in animals rather than humans) in birds in the early 21st century, and killed around half the humans that contracted it from birds. It comprises a protein coat (yellow) that encloses the viral genome on eight single strands (yellow/blue) of RNA (ribonucleic acid). The outer coat of the virus bears two types of protein, haemagglutinin (red), which the virus uses to enter host cells, and neuraminidase (green), which it uses to leave them. The disease is almost invariably fatal to poultry, but can be carried by many bird species, notably waterfowl, and as of early 2006 it had been found across much of Eurasia. The virus particle is around 100 nanometres across.