In the past 10 years, a deadly strain of bird flu has killed more than half of the people who got it. Now, there’s a new strain of bird flu circulating that can kill about a third of the humans who contract it. Whenever there’s a virus this lethal, there are always fears of a pandemic.That virus was H5N1. There is now a vaccine against it for poultry. But a new strain of bird flu has emerged and is causing alarm. This one is called H7N9. H7N9 doesn’t kill poultry or even make them sick, but it can be deadly when it is transmitted from a chicken to a person.Because of the genetic makeup of these strains of bird flu, it is difficult for them to spread from person to person. Since the H7N9 virus emerged a year ago, some 350 people have caught it. Thirty percent have died. Most had contact with poultry.The U.S. has developed a vaccine for H5N1 and is working on one for the newer strain of bird flu, but scientists don’t believe the new strain is any more threatening than H5N1. This virus has been circulating since 2003 and still doesn’t spread easily from person to person.