by Michaeleen Doucleff

In biology， you cant get much simpler than viruses.

They stick onto cells， pop open and then dump their genes inside to reproduce.

But the naming of viruses isnt so easy to follow.

Theres the new H7N9 spreading through China. And H5N1 is popping up in Southeast Asia. Last summer， H3N2 sickened dozens of people at country fairs.

Oh， lets not forget that 1）pain in the neck H1N1， or the swine flu， which swept through the U.S. in 2009.

What 2）in the heck do all these Hs and Ns mean？ H is short for 3）hemagglutinin. And N is short for 4）neuraminidase. More about those in just a moment.

But first， weve got to cover the 5）ABCs.

Influenza comes in three basic types： A， B and C. Those categories tell you a bit about how dangerous the viruses can be.

Influenza C causes the mildest disease. Although influenza B can make you just as sick as A， it has never triggered a worldwide 6）pandemic. Those have all come from the influenza A 7）strains.

Among other things， influenza A viruses are 8）fashionistas. They cover themselves in an array of accessories.

But instead of wearing 9）Dolce and Gabbana， influenza prefers “H & N，” aka hemagglutinin and neuraminidase.

Hemagglutinin and neuraminidase are little protein 10）spikes on the flus surface that help it invade cells.

Just like handbags and shoes， the spikes come in a variety of styles. There are 16 versions of hemagglutinin and 9 for neuraminidase.

Each flu picks out one style from the N rack and another from the H rack. The new bird flu in China， H7N9， covers itself in H7 and N9， while H1N1 is a bit more low-key and goes only for style No. 1 from each.

在生物學里，沒有什么比病毒更簡單的了。

它們粘附在細胞上，（侵入細胞內）脫殼，把自己的基因丟進去，然后分裂繁殖。

但是，病毒的命名卻沒那么容易理解。

H7N9這一新型病毒正肆虐中國。H5N1病毒則正在東南亞突然冒出來。去年夏天，H3N2讓鄉(xiāng)村貿易集市里的很多人感染致病。

噢，我們別忘了那讓人頭疼的H1N1，或者叫豬流感，它曾經(jīng)在2009年肆虐美國。

究竟這些“H”和“N”指的是什么？“H”是血細胞凝集素的簡寫，而“N”是神經(jīng)氨酸苷酶的簡寫。我們稍后再談這兩個家伙。

我們首先得來掃掃盲，介紹一些基礎知識。

流感分為三種基本類型：甲型、乙型和丙型。這些分類讓你知道各種病毒對生命的危害程度。

丙型流感引發(fā)的疾病最溫和。盡管乙型流感可能導致與甲型流感同等嚴重的病癥，但其未曾觸發(fā)全球性流行病。肆虐全球的流行病全都出自甲型流感菌系。

甲型流感病毒不但有許多特點，它還是個“弄潮兒”——身上“飾品行頭”十足。

不過，流感病毒沒有選擇穿戴“D & G”的設計品，反而喜歡“H & N”，也就是血細胞凝集素和神經(jīng)氨酸苷酶。

血細胞凝集素和神經(jīng)氨酸苷酶是附著在流感病毒表面的微小蛋白質尖狀物，能協(xié)助病毒攻擊細胞。

這些尖狀物就像手袋和鞋子一樣有著各種各樣的款式。目前，血細胞凝集素有16種變體，而神經(jīng)氨酸苷酶則有9種變體。

每一種流感都會從“N”類中挑出一種變體，從“H”類中挑出另一種變體。在中國爆發(fā)的H7N9新型禽流感病毒，以H7和N9兩種變體裝點自己，而H1N1則比較低調，都挑中了H類和N類的第一種。

11）All told， there are 144 different 12）subtypes of influenza A： from H1N1 to H9N16.

All of them exist in wild birds around the world， so they are all bird flus. But every now and then， one evolves the ability to infect other animals.

H1N1 is one of the more 13）promiscuous. It can infect birds， people， pigs and horses. H2N2， on the other hand， only makes its way into people and birds.

But all influenza A strains are 14）social climbers. Theyre always itching to try out new 15）living quarters.

So they 16）mutate. And they also do something even more 17）insidious： They swap accessories.

Geneticists call it 18）reassortment. And， historically， it signals trouble.

Say a bird gets infected with two viruses： H1N1 and H2N2. When the two viruses reproduce inside the birds cells， H1N1 can grab accessories from H2N2.

This creates a new strain that looks like H2N2 but can now infect people （because it started off as just H1N1）.

Thats what happened in 1957 during the Asian flu pandemic， says 19）computational biologist Richard Goldstein， from the National Institute for Medical Research in London.

Since H2N2 had never infected people before， our immune systems had never seen these accessories and thus， we had little defense against it. The new strain ended up killing roughly 70，000 Americans.

甲型流感總共有144種不同的亞型：從H1N1到H9N16。

所有這些亞型病毒都存在于世界各地的野生鳥類身上，因此它們都屬于禽流感。但是不時地，就會有一種亞型病毒進化出感染其他動物的能力。

H1N1是其中一種比較“饑不擇食”的亞型流感。它能感染禽類、人類、豬和馬。另一方面，H2N2卻只會感染人類和禽鳥。

不過，所有甲型流感菌系都是“趨炎附勢”的家伙，它們總在蠢蠢欲動，試圖開拓新的生活領地。

于是，它們學會了變異，也會使出一些更陰險的招數(shù)：它們會變裝！

基因學家把這種行為稱為基因重配。歷史經(jīng)驗告訴我們，這預示著麻煩要來了。

假設一只小鳥感染了兩種病毒：H1N1和H2N2。當這兩種病毒在小鳥的細胞內繁殖時，H1N1可以將H2N2的附著物取為己用。

這就創(chuàng)造出一種看似H2N2、但能感染人類的新型病毒變體（因為它當初就是H1N1）。

那正是1957年亞洲流感大流行時的情況，來自倫敦英國國家醫(yī)學研究所的計算生物學家理查德·高德斯坦如是說。

因為此前人類從未感染過H2N2病毒，所以我們的免疫系統(tǒng)根本沒有見過這些附著物，因而我們對它也就無從抵抗。這一新型病毒變體最終導致約七萬美國人死亡。

But the virus didnt stop there. “It then picked up H3 from birds in 1968 to cause the Hong Kong flu pandemic，” Goldstein says. That killed another 30，000 Americans.

What about more recent epidemics？

“The 2009 flu was a real mess，” he says. “The virus had some 20）segments from H3N2 circulating in humans from the 1968 pandemic. It had genes coming in from birds， an H1 coming from pigs and an N1 coming in from a completely different virus，” he says.“They all then mixed together in pigs.”

Scientists are still trying to figure out H7N9.

So far， it looks like the virus got its H from ducks， its N from migratory birds and a splash of other genetic material from chickens， 21）immunologists reported recently in 22）The Lancet.

But how this fashion bug found its way into people is still a mystery.

然而病毒并沒有就此罷休?！?968年，它從禽類中挑中了H3變體，從而引發(fā)了香港的流感疫情，”高德斯坦說。這次又有三萬美國人不幸病逝。

近幾年來的大流感又是怎樣的情況呢？

“2009年的流感疫情真的是一團糟，”高德斯坦說?！霸摬《編в胁糠謥碜?968年大流感時在人類中傳播的H3N2節(jié)片。它擁有源自禽類的基因、源自豬的H1變體以及一種來自某種截然不同的病毒的N1變體，”他說?！八鼈冏罱K都匯集到豬的身上?！?/p>

科學家們目前還在試圖摸清H7N9的底細。

免疫學家最近在《柳葉刀》雜志上發(fā)表報告稱：到目前為止，這一病毒看似從鴨類中獲得H變體，從候鳥中獲得N變體，以及從雞身上獲得一些其他基因物質。

然而，這一潮流病毒如何入侵人類，至今仍是一個謎。