[A] We try our best to develop a vaccine. [B] The cause of the flu epidemics arising so of
[A] We try our best to develop a vaccine.
[B] The cause of the flu epidemics arising so often in Asia.
[C] The general introduction of "BIRD FLU".
[D] The reason of the new strain of flu arising.
[E] The diseases caused by viruses threat human-beings.
[F] The connections between bird and viruses are complex.
1. ______
Human beings have known the diseases caused by viruses since thousands of years ago; and were fearful of them. The diseases are infectious hepatitis, polio, rabies, and AIDS. Now we can find easily that why the late Sir Peter Medawar, Nobel laureate in physiology or medicine, wrote "A virus is a piece of bad news wrapped in protein." Perhaps the most lethal virus in human history has been the influenza virus. About 21 million Americans and Europeans died of flu within 18 months in 1918 and 1919. It was an astonishing number. The outbreak in Hong Kong in 2003 of a potentially deadly strain of flu has attracted the world's attention once again on flu.
Flu viruses we call are RNA animal viruses. An individual flu virus resembles a rod studded with spikes composed of two kinds of protein. There are three "types" of flu virus, distinguished by their capsid protein, which is different for each type: Type A flu virus causes most of the serious flu epidemics in humans, and also occurs in mammals and birds. Type B and Type C viruses are only in to humans and rarely cause serious health problems.
2. ______
The new strain of flu arises not by mutation, but recombination. Viral genes are readily reassorted by genetic recombination, sometimes putting together novel combinations of H and N spikes unrecognizable by human antibodies specific for the old configuration. Viral recombination of this kind seems to have been responsible for the three major flu pandemics (that is, world wide epidemics) that have occurred in this century, by producing drastic shifts in Hand N combinations. The "Killer flu" of 1918, A (HIN1), killed 21 million people. The Asian flu of 1957, A (H2N2), killed over 100,000 Americans. The Hong Kong flu of 1968, A (H3N2), infected 50 million people in the United States alone, of which 70,000 died.
3. ______
It is proved that the new strains of flu usually start to develop in the far east. The most common hosts for influenza virus are ducks, chickens, and pigs, which in Asia often live in close proximity to each other and to humans. Pigs are subject to infection by both bird and human strains of the virus, and individual animals are often simultaneously infected with multiple strains. This creates conditions favoring genetic recombination between strains, producing new combinations of H and N subtypes. The Hong Kong flu, for example, arose from recombination between A (H3N8) [from ducks] and A (H2N2) [from humans]. The new strain of influenza, in this case A (H3N2), then passed back to humans, creating an epidemic.
4. ______
A potentially deadly new strain of flu virus, A (H5N1), has appeared in 2003, again in Hong Kong. Scientists paid great attention to this virus for two reasons. First, A (H5N1) represents a novel combination of H and N spikes, the sort of new combination that has in the past been associated with major flu epidemics. Second, unlike all previous instances of new flu strains, A (H5N1) passed to humans directly from birds, in this case chickens. Fortunately the bird flu virus does not appear to spread easily from person to person.
5. ______
A vaccine directed against the bird flu virus has not been developed. Such a vaccine will not be easy to produce. Public health scientists have injected the duck virus into laboratory ferrets, often used as test animals in flue research because they develop classic respiratory symptoms. Is the duck virus enough like bird flu that the ferrets develop
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