单项选择题

Scientists have discovered that influenza (流行性感冒) viruses taken from man can cause the disease in animals. In addition, man can catch the disease from animals. In fact, a great number of wild birds seem to carry the virus without showing any evidence of illness. Some scientists conclude that a large family of influenza viruses may evolved in the bird kingdom. There is even convincing evidence to show that virus strains (菌株) are transmitted from place to place and from continent to continent by migrating birds.
It is known that two influenza viruses can recombine when both are present in an animal at the same time. This raises the possibility that a human influenza virus can recombine with an influenza from a lower animal to produce an entirely new virus.
Research is under way to determine if that is the way that major new stains come into being. Another possibility is that two animal influenza strains may recombine in a pig, for example, to produce a new strain which is transmitted to man then. New virus may come into being ______.