If you've kept up with the news lately, you've probably heard dire warnings about avian flu, or bird flu. It's a highly infectious disease that has swept through the Asian bird population. In October of 2005, the disease reached Eastern Europe, most likely through migratory birds. By January of 2006, several people in Turkey had contracted the disease. Although the virus does not infect humans easily, more than half of the people who have contracted it have died. But what exactly is bird flu? How is it different from the seasonal flu that people experience every year? How does it threaten people? What are governments doing to stop its spread? Viruses and Influenza: An Overview A virus particle--or virion--is a microscopic packet that contains genetic material wrapped in a layer of protein. Some viruses also have a lipid membrane(脂质膜)around the protein coat. Unlike bacteria, they cannot reproduce on their own--they have to invade host cells. This process destroys cells and makes people sick. Viruses usually enter the bodies of animals and people through their mouths, mucus membranes or breaks in the skin. Then, they infect specific cells. For example, common cold viruses attack cells in the respiratory system. As they reproduce, they destroy their host cells, releasing copies of the virus to attack other cells. Some viruses are more stable than others, but in general they mutate frequently, sometimes making it difficult for doctors to treat them. Influenza is a specific type of virus that attacks the respiratory system. It can cause fevers, sore throats and congestion. If it attacks muscle cells, it can also cause muscle aches. There are three types of influenza virus--types A, B and C. Multiple subtypes exist within those types, and multiple strains(菌株)exist within each subtype. Like many viruses, influenza can mutate through antigenic drift(small changes that occur as it reproduces)or antigenic shift(major changes that create a new subtype of the virus). Avian Flu According to the United States Center for Disease Control and Prevention( CDC), birds carry every known subtype of influenza A. When scientists talk about avian flu, however, they usually mean varieties that exist mostly or entirely in birds--not in people. Most of the time, birds can't transmit the flu directly to people. They first infect pigs and other animals that can contract both human and avian flu strains. When the strains come into contact with one another, they create a new strain that infects humans. Many wild birds carry avian flu in their intestines(肠)and shed the virus in their droppings, but they don't usually get sick from it. Domesticated birds, however, can get sick when they come into contact with contaminated(污染的)water, feed or soil. Birds spread the disease to each other through their saliva, respiratory secretions and droppings. Avian flu is either low pathogenic(致病的)or high pathogenic. Low pathogenic strains cause very mild symptoms, like ruffled feathers and reduced egg production. High pathogenic strains, however, can be deadly--they often have a mortality rate approaching 100%. Birds that survive can continue to shed the virus in their droppings for ten days after recovering, which helps the virus continue to spread. Avian Flu H5N1 In 1997, health officials in Hong Kong reported a virulent(致命的)strain of avian flu. For the first time, it appeared that the virus moved directly from birds to people instead of moving through a second species. The virus caused typical flu symptoms, and it also lead to eye infections, pneumonia and acute respiratory distress. Tests confirmed that this strain of the virus, influenza A H5N1, was completely new to humans. Eighteen people infected with the virus were hospitalized, and six of them died. The Hong Kong government, alarmed at the potential thr