New York Times Researchers are not just worried about the virus popping up on American farms. Other types are causing trouble around the world. In the United States, the term bird flu has become synonymous with a particular virus that has devastated poultry and dairy farms over the past few years. But that virus, called H5N1, is not the only form of bird flu in circulation.
Concerned scientists are keeping a close eye on other types, including a fast-changing flu virus called H9N2.
In a study published in November, researchers in Hong Kong showed that over the last decade, this virus has acquired mutations that allow it to spread more efficiently among people and to cause more severe disease.
H9N2 is often discounted as a threat, because it causes only mild symptoms in poultry. But in people, especially children, the virus can cause more severe illness than the seasonal flu.