Avian influenza spreads in the air and in manure. Wild fowl often act as resistant carriers, spreading it to more susceptible domestic stocks. It can also be transmitted by contaminated feed, water, equipment and clothing; however, there is no evidence that the virus can survive in well cooked meat.
Cats are also thought to be possible infection vectors for H5N1 strains of avian flu (Kuiken et al, 2004).
Of the 15 subtypes known, only subtypes H5, H7 and H9 are known to be capable of crossing the species barrier from birds to humans. It is feared that if the avian influenza virus undergoes antigenic shift with a human influenza virus, the new subtype created could be both highly contagious and highly lethal in humans. Such a subtype could cause a global influenza pandemic, similar to the Spanish Flu that killed over 20 million people in 1918 (though a variety of sources quote average figures even higher, up to 100 million in some cases). Many health experts are concerned that a virus that mutates to the point where it can cross the species barrier (i.e. from birds to humans) will inevitably mutate to the point where it can be transmitted from human to human. It is at that point that a pandemic becomes likely.
The first cases of avian influenza passed from birds to humans was H5N1 in 1997 in Hong Kong. 18 people were infected, of whom 6 died. The outbreak was limited to Hong Kong. All chickens in the territory were slaughtered.
In January 2004, a major new outbreak of H5N1 avian influenza surfaced in Vietnam and Thailand's poultry industry, and within weeks spread to ten countries and regions in Asia, including Indonesia, South Korea, Japan and mainland China. Intensive efforts were undertaken to slaughter chickens, ducks and geese (over 40 million chickens alone were slaughtered in high-infection areas), and the outbreak was contained by March, but the total human death toll in Vietnam and Thailand was 23 people. In February 2004, avian influenza virus was detected in pigs in Vietnam, increasing fears of the emergence of new variant strains.
In North America, the presence of avian influenza was confirmed at several poultry farms in British Columbia in February 2004. As of April 2004, 18 farms have been quarantined to halt the spread of the virus. Two cases of humans with avian influenza have been confirmed in that region.
In August 2004 avian flu was confirmed in Kampung Pasir, Kelantan, Malaysia. Two chickens were confirmed to be carrying H5N1. As a result Singapore has imposed a ban on the importation of chickens and poultry products. Similarly the EU has imposed a ban on Malaysian poultry products. A cull of all poultry has been ordered by the government within a 10km radius of the location of this outbreak.
An outbreak of avian influenza in January 2005 affected 33 out of 64 cities and provinces in Vietnam, leading to the forced killing of nearly 1.2 million poultry. Up to 140 million birds are believed to have died or were killed because of the outbreak. As of April 17, the outbreak had claimed at least 50 human lives — most in Vietnam.
Vietnam and Thailand have seen several isolated cases where human-to-human transmission of the virus has been suspected. In one case the original carrier, who received the disease from a bird, was held by her mother for roughly 5 days as the young girl died. Shortly afterwards, the mother became ill and perished as well. In March, 2005 it was revealed that two nurses who had cared for avian flu patients have tested positive for the disease.
Avian influenza in humans can be detected with standard influenza tests. However, these tests have not always proved reliable. In March 2005, the World Health Organization announced that seven Vietnamese who initially tested negative for bird flu were later found to have carried the virus. All seven have since recovered from the disease.
Antiviral drugs are sometimes effective in both preventing and treating the disease, but no virus has ever been really cured in medical history. Vaccines, however, take at least four months to produce and must be prepared for each subtype.
In July 2004 researchers, headed by H. Deng of the Harbin Veterinary Research Institute , Harbin, China and Professor Robert Webster of the St Jude Children's Research Hospital , Memphis, Tennessee, reported results of experiments in which mice had been exposed to 21 isolates of confirmed H5N1 strains obtained from ducks in China between 1999 and 2002. They found "a clear temporal pattern of progressively increasing pathogenicity". 
In humans, it has been found that avian flu causes similar symptoms to other types of flu: