A、It causes a severe health problem. B、It leads to great economic losses. C、Humans may die of the infection without any sign. D、

END is a bird virus found in many parts of the world. Many kinds of birds carry it. But it especially affects chickens and a few others. It spreads very fast and there is no treatment. Many birds die without appearing sick.[26]The infection does not present a serious health threat to humans. But economic losses can be huge as birds are destroyed and trade is restricted to contain outbreaks. Experts say the easiest way to prevent the virus is to import birds from flocks that are disease-free. Vaccines are also used, although experts say the virus may sometimes cause deaths even in vaccinated flocks. The Global Invasive Species Database says signs of the disease may appear from two to fifteen days after a bird is infected. An infected hen lays fewer or no eggs, or eggs with thin shells. A sick bird may develop difficulty in breathing and twist its head and neck. It may run around in circles or not move at all.
    END spreads fastest among birds kept close together. The virus is spread through bird droppings and fluid from the nose, mouth and eyes.[27]To control outbreaks, experts advise quick destruction of infected flocks. They also advise limiting entry to farms and disinfecting vehicles as they come and go. People can also transport the virus on their shoes and clothes.[28]The virus can survive several weeks in a warm, moist environment. And there seems to be no limit if it is frozen. But ultraviolet rays in sunlight can kill it. To reduce injection risks, the Organization for World Animal Health warns against keeping any pet birds on a farm.
26. How would the bird virus END affect human beings?
27.What do experts suggest doing to control outbreaks of the bird virus?
28.What is the best way to kill the bird virus?