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.