The H5N1 bird flu virus has been endemic in southern China for almost 10 years and exists in healthy birds, the South China Morning Post reported, citing a new study of samples from more than 13,000 migratory birds. The study shows the virus has also developed into four distinct gene families, rendering the development of a single vaccine ineffective. Instead, bird flu must be controlled at its source. The study suggests the virus has probably been endemic in Fujian, Guangdong, Guangxi, Guizhou, Hunan, Yunnan and Hong Kong since 1996, when a single Guangdong goose virus was found. The long-term presence adds greatly to the risk of a pandemic.