Avian influenza or more commonly called as bird flu is an infection caused by avian (bird) influenza (flu) viruses. All birds naturally have these influenza viruses in their bodies. Wild birds have these viruses in their intestines but their bodies do not get infected or damaged by them. When passed on domesticated birds, avian influenza can cause sickness and death. Chickens, ducks, and turkeys are mostly hit by the transmission of avian flu among birds. Infected domesticated birds can pass on the virus to humans when they are eaten or consumed. *Migratory birds* infected with avian flu also become agents of viruses' transfer from one place to another.

H5N1 carrier birds pass on the virus to other birds through their saliva, nasal excretions, and feces. Birds get infected when they come in contact with these contaminated excretions even by just staying on a surface or ground where virus carriers have stayed. Cages, dirts, water, feeds, infected waterfowl or other poultries that have been exposed to the virus are the common sources of infection among domesticated birds.

Two forms of bird flu exist among domesticated poultries infected with avian influenza viruses. The low pathogenic form is almost invisible and undetected for it causes only very mild symptoms that can be mistaken as non-avian flu effects. Manifestations of low pathogenic avian flu infection include low egg production or ruffled feathers. The second form of avian flu is the high pathogenic avian flu disease. This form leads to multiple organ failures and damages among infected birds. About 90 to 100 percent of birds infected with this form die within two days upon contamination. The spread of this form of avian flu is more rapid among birds in flocks.

Albeit basically a matter among birds, humans are also susceptible to bird flu or avian flu infection. The first known cases of humans getting infected with bird flu happened in Hongkong in 1997. 18 people were recorded to have acquired respiratory illnesses after having contacted infected surfaces from poultry farms, the infection claimed six lives.

The virus' passage to humans that happened in Hongkong caused a global alarm. Health institutions worldwide raised concerns that the avian flu might be as damaging as the pandemic of flu that claimed about 20 million lives back in 1918. Reasonable enough, the fear and alarm are worth noting. What is essentially fearful is the idea that humans can be infected and actually die from a virus that is basically existing among birds only. The premise the Hongkong incident set is that the virus is mutating into something deadly for humans.

As of 2005, worldwide coordinated statistics recorded a number of 130 people having been infected of avian flu of which 67 have already died. In recent years, cases of human infection have been high in Asian countries. The World Health Organization believes that Asia is at higher risks since people live in close distance with domesticated birds like chicken and ducks. Animal domestication and poultry raising are common among many Asian countries who are agricultural by nature. What worries many is the common fact that humans have little or even a lack of immunity to the virus known to be infecting only (until the 1997 Hongkong incident) the birds.

An even higher cause of alarm has been found in recent studies on the H5N1 virus. Recent researches have shown that H5N1 strains have become more deadly among chickens and mice. This mutation is also found to be making cats or feline susceptible to the infection. H5N1 is also found to be resistant to some of the drugs used to treat flu (such as amantadine). New strains of the virus are also said to be possibly emerging especially in Asian countries.

What is now being focused on by scientists and health experts around the world is the prevention of human-to-human transmission which is believed to be the possible case that happened in Thailand in 2004. If it becomes widespread, human-to-human transmission is believed to be harder to contain that bird-to-human transmission. Experts say that the virus' continual change will make it hard for the human body to naturally develop immunity against the infection. The H5N1 virus is found to be changing over time through the changes happening in their structure called antigenic drift and antigenic shift. This continual change of the virus disables the immune system to respond accordingly whenever the flu virus enters the body. The only possible solution seen by experts is for infection-prone people to have a yearly immunization with up-to-date anti avian virus strains. But the big problem is, until now there is no medical treatment available to combat the H5N1 virus, making the yearly immunization update a far-fetched option.