The HPAI H5N1 strain of avian influenza was first identified in Hong Kong in 1997 and behaves as an HPAI strain. Starting in 2003 the virus spread widely among poultry flocks in a number of south-east Asian countries. In 2005, 2006 and 2007 it spread westward into Europe and Africa along the migration paths of wild birds, particularly waterfowl. No specific cause for the February 2007 outbreak of HPAI H5N1 in England has yet been identified, but it has been suggested the outbreak may be linked to frozen uncooked turkey meat imported from Hungary where there had been an outbreak of HPAI H5N1 a month earlier. The meat did not come from birds who were sick and dying, but from birds thought to have subclinical infections or birds that were infected, but showed no signs of illness. Avian influenza virus has been shown to survive in a variety of environmental extremes including frozen lake water, thus the virus may survive in frozen uncooked product. Some 50 non-domestic species are capable of being infected with the HPAI H5N1 virus, but aquatic birds play a major role in spreading the virus from one country to another.