According to the United States Center for Disease Control and Prevention (CDC), birds carry every known subtype of influenza A. When scientists talk about avian flu, however, they usually mean varieties that exist mostly or entirely in birds -- not in people. Most of the time, birds can't transmit the flu directly to people. They first infect pigs and other animals that can contract both human and avian flu strains. When the strains come into contact with one another, they create a new strain that infects humans.
Many wild birds carry avian flu in their intestines and shed the virus in their droppings, but they don't usually get sick from it. Domesticated birds, however, can get sick when they come into contact with contaminated water, feed or soil. Birds spread the disease to each other through their saliva, respiratory secretions and droppings.
Avian flu is either low pathogenic or high pathogenic. Low pathogenic strains cause very mild symptoms, like ruffled feathers and reduced egg production. High pathogenic strains, however, can be deadly -- they often have a mortality rate approaching 100%. Birds that survive can continue to shed the virus in their droppings for ten days after recovering, which helps the virus continue to spread.
Poultry farmers can protect their birds from avian flu by following biosecurity practices. In an effort to slow the spread of various strains of avian flu, farmers disinfect their clothing and shoes as well as their farm equipment. They also quarantine birds that may be infected and keep domestic birds away from wild birds.
Farmers aren't just protecting their birds or their livelihood -- they're also protecting human health. In rare cases, avian flu can spread to humans. People have no immunity to avian strains of flu, so these viruses can be especially deadly. In the next section, we'll look at avian flu H5N1, the high pathogenic virus that has made the headlines by infecting and killing people in Turkey and Southeast Asia.