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Scientists have estimated that influenza viruses are taken from man can cause the disease in animals. In addition, man can catch the disease from animals. In fact, a great amount of wild birds seem to carry the virus without showing some evidence of illnesses. Some scientists conclude that a large family of influenza viruses may have evolved in the bird kingdom, a group that has been on the earth for 100 million years and is able to carry the virus without attracting the disease. There is even convincing evidence to show that virus strains are transmited from place to place and from continent to continent by migrating birds. It is known that two influenza viruses can recombine when all are present in an animal at the same time. The result of such recombination is a great variety of strains containing different H and N spikes. This rises the possibility that a human influenza virus can recombine with an influenza virus from a lower animal produce an entirely new spike. Research is underway to determine if that is the way that major new strains come into being. Other possibility is that two animal influenza strains may recombine in a pig, for example, to produce a new strain which is |