How humans get bird flu?

Migratory waterfowl, and ducks in particular, carry the viruses that cause bird flu. Often unaffected themselves, the host birds spread the infection to susceptible species, especially domesticated chickens, turkeys and geese, resulting in severe epidemics that sicken and kill large numbers of birds - sometimes in a single day.
Avian viruses generally don't affect humans, but in 1997, an outbreak of bird flu in Hong Kong infected 18 people, six of whom died. Since then, human cases of bird flu have been reported in Asia, Africa, the Pacific, Europe and the Middle East. Most were traced to contact with infected poultry or surfaces contaminated by sick birds.
Often, flu viruses that cross from animals to humans originate in areas where people live in proximity to chickens and pigs. That's because pigs are susceptible to infection with both avian and human viruses and so are an ideal "mixing bowl" in which viruses can exchange genes.
But at least some bird flu viruses don't need a third party. Instead, they shuffle and rearrange their genetic material directly in humans. That seems to be the case in most instances of human-acquired bird flu. People become sick after direct contact with infected birds or bird-contaminated surfaces, not from contact with pigs or other animals.

how transmitted the bird flu.

Direct bird-to-human transmission works like this:

Wild birds shed the virus. Infected migratory waterfowl, the natural carriers of bird flu viruses, shed the virus in their droppings, saliva and nasal secretions.


The virus spreads to domesticated birds. Domestic poultry become infected from contact with these birds or with contaminated water, feed or soil. They may also catch the disease the same way humans contract conventional flu - by inhaling the airborne virus. Bird flu spreads quickly and lethally within domestic flocks and is inadvertently transported from farm to farm on tractors and other equipment, on cages, and on workers' shoes and clothing. Heat destroys the virus, but it can survive for extended periods in cool temperatures


Markets provide pathways to humans. Open-air markets, where eggs and birds are often sold in crowded and unsanitary conditions, are hotbeds of infection and spread the disease into the wider community. Cockfighting, common throughout much of Asia, has also been implicated in the spread of bird flu because fighting roosters are often trucked long distances and smuggled across borders. At any point along the way, humans may pick up the virus through close contact with sick birds or contaminated surfaces.

Casuative agent of Bird flu (avian influenza).

All influenza viruses are divided into three types - A, B or C - depending on the virus' structure.
Type A is responsible for lethal influenza pandemics, whereas
Type B causes smaller, localized outbreaks.
Both types A and B are also responsible for the seasonal flu that affects many people every year. Less common and more stable than other strains, type C has milder symptoms.
Influenza B and C are usually found only in humans. But type A influenza infects both people and animals, including birds, pigs, horses, whales and seals.
Numerous influenza A subtypes exist, and they can combine to form even more subtypes, some of which normally are specific to a single species. At least 15 flu subtypes affect birds, the most virulent of which is H5N1. Until recently, avian subtypes have rarely been found in humans or in animals other than pigs.
Type A influenza viruses are further divided into strains, which are constantly evolving. And it is exactly this - the ability of influenza viruses to change their genetic makeup and to swap genes indiscriminately - that makes them so unpredictable and potentially deadly.