Why are half of my white birch and pear trees dying?

Both birch and many pears are susceptible to a fatal tree disease called fire blight-and it spreads. There are resistant cultivars, but once it starts in an orchard, usually the only thing to do is remove the trees and spray the remaining trees.
Why are half of my white birch and pear trees dying?
We need a few more clues before any of us can help. What zone are you in, do you see other trees like this in your neighborhood ? RScott