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On March 31, 2013, the Centre for Health Protection (CHP) of the Department of Health of China announced that three people in two widely separated locations in China had been infected with H7N9, a relatively new strain of avian, or "bird," flu. Two people infected with the virus in Shanghai, and 27-year-old man and an 87-year-old man, died of the infection.
Do these three cases of a previously unreported strain of the flu presage a pandemic spreading around the world? Or are these deaths likely to be among the last caused by the virus? We'll get to those questions in a moment, but first it helps to clarify some of the terminology of the disease.
What Is "Bird" Flu?
Bird flu, or avian flu, as its name suggests, is an influenza virus that occurs in birds. There are also strains of the virus denoted swine flu, horse flu, dog flu, and human flu. Not all flu viruses make the jump from their animal hosts to people, although in at least three cases, H7N9 has.
Bird flu is an "influenza type A" virus. All strains of influenza type A are found in birds, although the "A" doesn't refer to "avian." The important difference between influezavirus A, influenzavirus B, and influenzavirus C is the speed at which viruses mutate so that the immune system cannot build a permanent defense against them. Type A flu viruses mutate faster than type B flu viruses, and type B flu viruses mutate faster than type C flu viruses.
Why Is Bird Flu So Often in the News?
The reason avian, or bird, flu gets so much attention from doctors, public health officials, and the media is that it is always changing. From year to year, the flu virus changes enough that last year's flu shot or last year's case of flu fails to protect against the newly emerging strain. Just as vaccine manufacturers have to guess what to put in their flu shots, the immune system can only muster very generalized defenses against the infection. And bird flu is particularly hard to vaccinate against.
What Do the "H" and "N" in H7N9 Stand For?
Both "H" and "N" in the name of a flu virus identify the antigen activated by the virus. An antigen is an "antibody generator," a substance that activates the immune system to respond to a disease. There are 17 different H antigens and 9 different N antigens known to science, but not all combinations have actually appeared outside the laboratory. These are the first known cases of H7N9 in people.
Is Bird Flu Really Spread by Birds?
Of course, avian influenzaviruses are spread from place to place by migrating birds. Because the virus that killed millions during the Great Flu Epidemic of 1918 was an avian flu virus, many people are especially wary of the emergence of any new kind of bird flu. The better adapted a virus is to living in birds, that is multiplying in birds but not killing them before they have a chance to fly to another location, the deadlier it typically is in humans.