(CNN) -- Below is a list of key questions and answers regarding the bird flu that is sweeping across Asia and has arrived on Europe‘s doorstep. What is bird flu (avian influenza)? 旳vian influenza is an infectious disease of birds caused by type A strains of the influenza virus. The disease, first identified in Italy more than 100 years ago, occurs worldwide. 旳ll birds are thought to be susceptible to the avian influenza, though some species, such as wild ducks, are more resistant than others. Domestic poultry, such as chickens or turkeys, are particularly susceptible. 旾nfection triggers a wide spectrum of symptoms in birds, ranging from mild illness to a highly contagious and rapidly fatal disease resulting in severe epidemics. 旾n severe cases, the flu is characterized by a sudden onset of severe illness, and rapid death, with a mortality that can approach 100 percent. Have humans come down with bird flu? 旳vian influenza does not normally infect species other than birds and pigs. But humans came down with the bird flu in Hong Kong in 1997, when the H5N1 strain infecting 18 humans, 6 of whom died. 昑hen, people became infected after coming into close contact with live infected poultry. 旼enetic studies showed the virus jumped directly from birds to humans, and caused severe illness with high mortality. 旽ong Kong‘s entire poultry population, estimated at around 1.5 million birds, was destroyed within three days. This is thought to have averted a pandemic. 昑he World Health Organization has said the H5N1 bird flu virus is responsible for 60 deaths in Indonesia, Vietnam, Thailand and Cambodia. They suspect people became ill after coming into contact with chicken feces. 旾n September last year, Thailand‘s Ministry of Health announced possible human-to-human transmission in a family cluster. The WHO says that evidence to date suggests the transmission was limited to family members and no wider community spread occurred. Why is H5N1 of particular concern?Of the 15 avian influenza virus subtypes, H5N1 is of particular concern because: 旾t mutates rapidly and seems to acquire genes from viruses infecting other animal species. 旾t can cause severe disease in humans. 旴irds that survive infection excrete virus for at least 10 days, orally and in feces, helping spread the virus at live poultry markets and by migratory birds. 昑he more birds that come down with bird flu, the greater the opportunity for direct infection of humans. 昑he more humans get infected, the greater the likelihood people can become infected with both human and bird flu strains. 旽umans could then serve as a "mixing vessel" for a new type of virus that could easily be transmitted from person to person. Such an event would mark the start of an influenza pandemic. What are the symptoms, can it be tested and how do you treat it?昗hen humans came down with H5N1 bird flu in Hong Kong in 1997, patients developed symptoms of fever, sore throat, cough and, in several of the fatal cases, severe respiratory distress secondary to viral pneumonia. 昉reviously healthy adults and children, and some with chronic medical conditions, were affected. 昑ests for diagnosing all influenza strains of animals and humans are rapid and reliable. 旳ntiviral drugs, some of which can be used for both treatment and prevention, are clinically effective against influenza A virus strains in otherwise healthy adults and children, but have some limitations. 旳t least four months would be needed to produce a new vaccine, in significant quantities, capable of conferring protection against a new virus subtype. -- Compiled from a World Health Organization fact sheet. | 
