All birds are supposed to be susceptible to the avian influenza, but particularly domestic poultry. The only subtype that can cause severe illness to people is Influenza A /H5N1 virus, initially it affects chickens, ducks and other birds by the process of mutation they can become highly pathogenic. If the bird flu virus recombines with a human flu virus and mutate it may become possible the transmission from human to human as happened in Asia, Indonesia, Vietnam, Cambodia Thailand. The disease may affect the birds in two form: a mild one and a highly lethal one.

The lethal form manifests as a severe disease, rapid contagious and a high mortality rate in 48 hours. It has been named"chicken Ebola" because of the massive internal haemorrhaging. All 16 HA and 9 NA subtypes are known to infect waterfowl, but in wild birds routine testing will always find new viruses those are inactive viruses. Highly pathogenic viruses have a genetic mark which distinguishes them from other viruses, by combining sets of amino acids the low pathogenic viruses transform in highly pathogenic ones.

Avian influenza viruses are easily transported from farm to farm even to new geographical areas by migratory birds and by contaminated people, vehicles, equipment, feed, and cages. Viruses survive for quite long in low temperatures, in high temperatures viruses have been shown to survive fewer days. The World Organisation for Animal Health (OIE) recommends vaccination against outbreaks. Control measures are of all infected or exposed birds, proper disposal of carcasses, the quarantining and rigorous disinfection of farms, and the implementation of strict sanitary, or biosecurity, measures.

Outbreaks are difficult to control and present a high risk of infection, poverty is a determining factor for transmission of bird flu when butchering and preparation of poultry meat for cooking. Restrictions on the movement of live poultry, both within and between countries, are another important control measure. In small rural areas an outbreak is hard to be detected because usually people are disinformed and tend to think that thing aren't so bad and that it could only be a natural death, not even thinking of avian influenza so they do not alert the authorities. The owners may even want to protect their birds when culling because if the avian influenza virus is found the birds will be killed and the farmers won???t be recompensated. Control is far more difficult under poultry production systems in which most birds are raised in small backyard flocks scattered throughout rural or periurban areas.