Bird Flu
The H5N1 strain of influenza(H5N1 类禽流感)-often referred to as bird flu-was first known to have spread from chickens to humans in 1997. Since 2004 it has attacked Asian poultry farms (家禽饲养所), and had a 7% death rate in the first 70 people who were known to have been infected. Health authorities fear this strain, or its descendents (变种), could cause a fatal new flu with the potential to kill billions.
Flu has been a regular disaster of humanity for thousands of years. The flu viruses are a large family, each possessing a mere 10 genes encoded in RNA (核糖核酸). All of the 16 known groups come first from water birds, especially ducks and gulls (鸥). The virus is well adapted to their immune systems, and does not usually make them very sick. This leaves the animals free to move around and spread the virus.
Violent virus
But every now and then a bird flu virus is transmitted to all animal whose immune system the virus is not adapted to. If the bird flu from a forest bird spreads to chickens, it causes an average disease but can readily change to a more severe strain. Just such a strain of HSN1 flu has hit large chicken farms in East Asia.
The situation is serious because, in 1997, scientists found for the first time that H5 flu is capable of infecting humans. It was found in 18 people, six of whom died. All the poultry in Hong Kong were destroyed to stop the threat. But it continued to circulate, especially in China.
There were further human cases in China in 2003. Then in early 2004 Vietnam (越南) reported widespread poultry outbreaks and some human cases. Cambodia (柬埔寨) and Thailand denied the outbreaks in the beginning but admitted the fact later. So did China.
A mass poultry selection stopped the outbreaks by March 2004, by which time 23 people had died. But the virus went on, most probably in ducks. But scientists think that we should not blame its persistence and spread in the region all on wild birds. The outbreaks started again in summer 2004, and by mid-April 2005 had caused a total of 51 human deaths, all in Thailand and Vietnam.
Making the jump
The two or three flu virus families that have made the jump to humans mostly cause slight disease, because they have adapted to our immune systems. A yearly winter flu suffers most of the world. But it is not totally benign (良性的). About 700,000 people around the world die of it each year, mainly the very old, very young and the weak.
Common flu vaccines(疫苗) are increasing in popularity, although flu evolves so fast that we need new flu vaccines every year. In 2004 an unexpected shortage of vaccine in the US indicated the weakness of the vaccine supply, which is produced by very few manufacturers. As New Scientist predicted, it took great efforts to limit available supplies to those most at risk of serious illness, preventing extra deaths.
But flu is most deadly when it first makes the jump to people because the virus had no opportunity to adapt itself to our immune systems. H5N1 has continued to infect humans as the outbreak in poultry has increased, with an apparently high death rate. It has so far been hard to infect human beings, and has not spread readily between people. If this viral strain should acquire that ability of attacking us, it could become a lethal pandemic (流行性疾病)-the name for a wide-spread disease that spreads worldwide.
Deadly widespread
In 1918, a virulent (有毒菌株) flu strain appeared in humans and killed 50 million people within a few months.
There have also been two less catastrophic(悲惨的) pandemics. The so-called "Asian" flu of 1957 caused between one and four million deaths, while 1968's "Hong Kong" flu with about half the estimated deadliness of the Asian flu--caused one to two million deaths. Both of these were human flu viruses which

A. Y
B. N
C. NG