Transmission and infection of H5N1  Transmission and infection of H5N1 from infected avian sources to humans has been a concern since the first documented case of human infection in 1997, due to the global spread of H5N1 that constitutes a pandemic threat. Infected birds pass on H5N1 through their saliva, nasal secretions, and feces. Other birds may pick up the virus through direct contact with these excretions or when they have contact with surfaces contaminated with this material. Because migratory birds are among the carriers of the H5N1 virus it may spread to all parts of the world. Past outbreaks of avian flu have often originated in crowded conditions in southeast and east Asia, where humans, pigs, and poultry live in close quarters. In these conditions a virus is more likely to mutate into a form that more easily infects humans. A few isolated cases of suspected human to human transmission exist, with the latest such case in June 2006 (among members of a family in Sumatra). No pandemic strain of H5N1 has yet been found. H5N1 vaccines for chickens exist and are sometimes used, although there are many difficulties, and it's difficult to decide whether it helps more or hurts more. H5N1 pre-pandemic vaccines exist in quantities sufficient to inoculate a few million people and might be useful for priming to "boost the immune response to a different H5N1 vaccine tailor-made years later to thwart an emerging pandemic". H5N1 pandemic vaccines and technologies to rapidly create them are in the H5N1 clinical trials stage but can not be verified as useful until after there exists a pandemic strain. Avian flu virus can last indefinitely at a temperature dozens of degrees below freezing, as is found in the northern most areas that migratory birds frequent. Heat kills H5N1 (i.e. inactivates the virus). Influenza A viruses can survive:  While cooking poultry to 70 °C (158 °F) kills the H5N1 virus, it is recommended to cook meat to 74 °C (165 °F) to kill all foodborne pathogens. Inactivation of the virus also occurs under the following conditions:  Ordinary levels of chlorine in tap water kill H5N1 in public water systems. To kill avian flu viruses,   H5N1 "can remain infectious in municipal landfills for almost 2 years. The two factors that most reduced influenza survival times were elevated temperature and acidic or alkaline pH."