阅读理解 As students across the country have their yearly flu shots(打流感疫苗)，doctors across the world are preparing against a possible outbreak of bird flu that few current vaccines(疫苗)will guard against．The likelihood of a bird flu spreading among humans is very high, the World Health Organization warned recently．European countries worry about bird flu very much, since many migratory birds(候鸟)start to fly back from the North． There are at least 15 different types of bird flu virus around the world．The possible outbreak may be caused by a virus known as H5N1．It was first found to have spread from a bird to a human in Hong Kong in 1997．In addition to Asia, this bird flu has been found among birds in Romania, Turkey, Russia and Kazakhstan． The outbreak in 1997 in Hong Kong was quickly controlled at a cost of 18 people ill and six dead．About 1.5 million chickens were collected in an effort to remove the source of the virus．But scientists say that the virus can change itself rapidly．Also, if the virus mixes with the human flu virus, it could be spread through person-to-person contact in the same way as ordinary human flu is spread．Bird flu can cause fever, coughing, sore throats and muscle aches in humans．With the introduction of vaccines, flu is not as deadly as it used to be．But throughout history it was a huge cause of death around the world．The outbreak of Spanish flu in 1918-19 killed about 40 to 50 million people worldwide． Currently there is no vaccine against bird flu that can be used on humans．Scientists are working to develop one．This will probably take several months to complete and may not be ready in time to stop a widespread outbreak of the disease． | 
