natural hosts for a range of influenza A viruses. Occasionally, viruses are transmitted from these species to other species, and may then cause outbreaks in domestic poultry or, rarely, in humans. H5N1 (Avian flu) In February 2004, avian influenza virus was detected in birds in Vietnam, increasing fears of the emergence of new variant strains. It is feared that if the avian influenza virus combines with a human influenza virus (in a bird or a human), the new subtype created could be both highly contagious and highly lethal in humans. Such a subtype could cause a global influenza pandemic, similar to the Spanish flu or the lower mortality pandemics such as the Asian flu and the Hong Kong flu. From October 2004 to February 2005, some 3,700 test kits of the 1957 pandemic influenza virus, H2N2, were accidentally spread around the world from a lab in the U.S. In May 2005, scientists urgently called upon nations to prepare for a global influenza pandemic that could strike as much as 20% of the world's population. In October 2005, cases of the avian flu (the deadly strain H5N1) were identified in Turkey. EU Health Commissioner Markos Kyprianou said: "We have received now confirmation that the virus found in Turkey is an avian flu H5N1 virus. There is a direct relationship with viruses found in Russia, Mongolia, and China." Cases of bird flu were also identified shortly thereafter in Romania, and then Greece. Possible cases of the virus have also been found in Croatia, Bulgaria, and the United Kingdom. By November 2007, numerous confirmed cases of the H5N1 strain had been identified across Europe. However, by the end of October, only 59 people had died as a result of H5N1, which was atypical of previous influenza pandemics. Avian flu cannot be categorized as a "pandemic" because the virus cannot yet cause sustained and efficient human-to-human transmission. Cases so far are recognized to have been transmitted from bird to human, but as of December 2006 there had been few (if any) cases of proven human-to-human transmission. Regular influenza viruses establish infection by attaching to receptors in the throat and lungs, but the avian influenza virus can attach only to receptors located deep in the lungs of humans, requiring close, prolonged contact with infected patients, and thus limiting person-to-person transmission. Zika virus An outbreak of Zika virus began in 2015 and strongly intensified throughout the