Species Transfer
Bird flu describes strains of influenza that originate from those found in birds. As the National Institute of Allergy and Infectious Diseases explains, many wild birds are hosts to strains of influenza (typically type A influenza viruses). These birds are able to tolerate the viruses with few, if any, ill effects. However, in some cases the viruses are able to infect domesticated ducks. Domesticated ducks then allow for the virus to be transferred to other farm animals, such as chickens and pigs. Swine represent an opportunity for the evolution of these avian influenza viruses, because swine can also be a host for other forms of influenza that can infect humans. When a pig is infected with two different viruses, sometimes these viruses exchange genetic information, which can allow the bird flu virus to gain the ability to infect humans as well.
Bird-To-Human Transmission
The Mayo Clinic says bird flu is able to be transmitted through contact with the feces from an infected bird or secretions from its nose, eyes and mouth. As a result, places such as open-air markets--where a large number of birds and eggs are brought together and sold--allow for the rapid spread of bird flu throughout a community. Bird feces encountered in the wild (such as pigeon droppings in urban areas) also represent a potential route of human infection. The Food and Drug Administration also notes that properly cooked poultry and eggs cannot cause transmission of bird flu.
Human-To-Human Spread
One of the greatest fears that scientists have is that bird flu will be able to spread efficiently from one person to the next. The Centers For Disease Control and Prevention notes that there are some cases of human-to-human transmission of bird flu, but that these have been very limited. In Thailand and Indonesia, for example, there has been limited human-to-human spread, but it was within families that had very close contact with infected people.
