Poultry blood samples taken by Hanoi Animal Health Dept at Long Bien Market to test for avian flu. Vietnam suffered outbreaks of avian flu in 2004-5. Avian flu, also known as bird flu, is caused by viruses that occur naturally among wild birds. The virus is carried by birds in their intestines. While the wild birds usually do not get sick, the flu has caused some domesticated birds to become fatally ill. The infected birds; including chickens, ducks, and turkeys, began to spread the viruses to humans in 1997.