I'm confused about the Bird Flu virus (avian influenza)?

Okay, so I read that avian influenza is an infection thats caused by influeza that occurs naturally in birds. And that the Bird Flu virus thats become an epidemic an some areas is a strain from the bird flu. That virus is referred to as H5N1
but I'm confused:

"Avian influenza - commonly called "bird flu" - is an infection caused by influenza viruses that occur naturally in birds.
Wild birds can carry the viruses, but usually do not get sick from them. However, some domesticated birds, including chickens, ducks, and turkeys, can become infected, often fatally.One strain of avian influenza (bird flu), the H5N1 virus, is endemic in much of Asia and has recently spread into Europe. Avian H5N1 infections have recently killed poultry and other birds in a number of countries."

does that mean that the influenza that occurs naturally in birds isn't usually harmful to humans, but that that particular strain of it is whats so contagious and harmful?

PLEASE HELP! only my 2nd research paper

There are flu viruses endemic to birds and mammals, and sometimes only certain species of birds or mammals. Because the cells of different species share similar tissues and biochemical pathways, there is always potential for a virus to switch to a different species. H5N1 virus strain has a structure that makes it easy for this virus to mutate into a strain that can spread from person to person. The H5 strain has caused human pandemics in the past. Right now, the virus can spread from birds to humans if you slaughter or raise an infected bird. There are several instances where it appears the flu was passed from one person to another in the same family. There's tons of info ath this link: