Both seasonal flu, a disease like a very bad cold, and much larger pandemics, like the one caused by H7N9（禽流感） in 2013, are the result of a race between the flu virus  and the immune system which can protect people from illness. Here's how it works.

    Viruses, which cause pandemics, have two important proteins "haemagglutinin(H) and neuraminidase(N).H helps the virus go into a cell .N helps new virus break out of that cell .They are parts of a virus that may be known and dealt with by the immune system

    A flu virus attacks a host cell in order to reproduce itself. But the virus created out of it is quite different now. The virus's genetic material is copied with low fidelity. That difference produces virus which immune system does not always know and deal with immediately. Each year's seasonal virus is therefore a little bit different, and thus requires an updated vaccine to prevent people from getting that disease.

    Pandemics viruses have bigger changes in this process .Many hosts' defences are unprepared for such big changes, which happen three or four times a century .The 1918 pandemics was caused by viruses carrying H1 and N1.Anotber two pandemics in 1957 and 1968 were caused by viruses carrying H2and H3 together with N2.They are numbered by scientists. Much of this change goes on outside humans. A flu is usually from sick binds. Sometimes a bird-flu appears with materiel that enable it to spread among people .The H7N9 is recently worrying scientists in the health center.

    The flu virus sometimes jumps directly from a bird(often a chicken) to a person, but more usually passes through a pig The cells in bird and human are built differently, It means us that the virus needs different sets of material to attack them .Pig cells have features of both so that it can act as a platform in which the virus can fit from one to the other .Even after it has made humans ill, a virus cannot go ca to cause a pandemic unless it also gets the ability to pus easily between people, Luckily, this is something H7N9 has yet to do.

Clever people always bring us new things. They make our life easy. Let's have a look!

The 9th day of the 9th lunar month is the traditional Chongyang Festival, which falls on Oct. 4th this year. "Chong" means "double" in Chinese. 1 , as double ninth was pronounced the same as the word to express "forever", Chinese ancestors considered it a good day worth celebration. That's 2 ancient Chinese began to celebrate this festival long ago. 

People follow several customs on Double Ninth Festival. For example, people often 3 the cornel (山茱萸) on their arms or heads because they believe this plant can prevent diseases and avoid disasters. Besides, people like to climb 4 on this day, so Double Ninth Festival is 5 Mountain Climbing Festival. The 9th lunar month often has autumn skies and fresh air. It is a good time 6 sightseeing. When people stand on a high mountain, they will have a good view of a blue sky and dark green mountains, which makes 7 carefree and joyful. Chongyang Cake is also known as "flower cake". It is said that the 8 was originally (最初) prepared after autumn harvest, because farmers wanted to have a 9 of what was just in season. Then gradually it became the cake for people to eat on Double Ninth Festival. Double Ninth Festival is a festival for the 10 . People get together to show their love and concern for their parents and their grandparents. 

①If your family speak English at home, then your child will naturally start to pick it up(自然地开始学习它). From an early(早的)age, children pick up what they hear every day in their home. In fact, if you can speak English, you can do something to teach your child English.

②Talk with your child. Talk with your child in English often. When talking with you, your child can pick up words and sentences (单词和句子). Children learn from the people around them.

③Read with your child. Reading English books is a good way for children to learn English. You can use pictures to help your child. What you read is very important. Always give your child the right(合适的 English books. Find him the right pictures, too.

④____ Children learn first from parents. so here are two tips(建议) for you. Your child is always listening to you, so make every talk a good learning opportunity(机会). English books are the best tool(工具) you have to teach English. Pictures can help your child to learn well.

⑤ Learning: English is s difficult I but the best time to learn English is when your child is small.

 If you have no special plans for your holidays, why not spend your time helping others? Nowadays, more and more young people in China volunteer to spend their school holidays working for others. And they don't do it for money. Here are some of their plans.

 Li Mei,17

I'm going to help the old people at the Old People's Home. I think they are lonely and hope someone to talk with and share stories. Also, they need to be taken good care of. I will help them do some washing and reading.

 Lin Wei,18

 This summer,I'm going to work for Special Help Program as a volunteer. I will visit a disability(残疾) center in my city. I plan to prepare some books and CDs for them. To cheer them up,I decide to play some games and share stories with them.

 Jin Fei,16

I'm going to teach kids who have trouble reading. I' ll work for a program called Reading For Life. Every day,I'm going to help kids choose and read books that they like. I love children and I want to be a teacher, so this is going to be a great experience for me.