1. Viruses are the smallest and simplest living things on the planet, but they have terrible effects on the other living things. 

病毒是地球上最小又是最簡(jiǎn)單的生物體，但是對(duì)其他生物體有可怕的作用。

2. A virus can be as minor as a common cold, or deadly like the AIDS virus.

一種病毒的作用可以像普通感冒一樣輕微，或像艾滋病病毒那樣致命。

3. But all the viruses have one common purpose: to reproduce. 

但是，所有病毒有一個(gè)共同的目的：繁殖。

4. A virus is structured in such a way that it can attach itself to a cell. Then the viral DNA takes over the healthy cell and uses it to make more viruses. 

病毒的結(jié)構(gòu)是這樣的：它能依附在細(xì)胞上，然后病毒的DNA攻擊健康細(xì)胞，用它來(lái)制造更多的病毒。

5. Once the virus multiplies, the host cell explodes and releases new viral cells that attach themselves to other healthy cells, starting the cycle over again.

一旦病毒繁殖，宿主細(xì)胞激增并釋放新的病毒細(xì)胞，依附在其他健康細(xì)胞上，再次開(kāi)始這樣的繁殖循環(huán)。

6. Certain viruses are so harmful that viral epidemic can be considered a natural disaster. 

某些病毒非常有害，以至于病毒流行可以被看作為一場(chǎng)自然災(zāi)害。

7. In 1918, an influenza pandemic and epidemic over a large area struck every major U.S. city.

在1918年，一場(chǎng)大面積的流感大流行襲擊了美國(guó)每一個(gè)大城市。

8. Scientists have developed vaccines to combat some harmful viruses. 

科學(xué)家們已經(jīng)開(kāi)發(fā)疫苗來(lái)抗擊一些有害的病毒。

9. Vaccines work by infecting a person with an innocuous or harmless form of the virus. 

疫苗通過(guò)用無(wú)害或不致害的病毒形式感染人，因而起到抗病毒的作用。

10. Parts of the immune system known as memory cells are then able to recognize and destroy the virus if it ever enters the body again.

如果那種病毒再次進(jìn)入人體，被稱為記憶細(xì)胞的免疫系統(tǒng)一部分就能夠識(shí)別和破壞它。

11. Not all viruses can be fought with vaccines. Certain types of viruses continually mutate or change, making it impossible to create vaccines that will protect against all the mutations.

并非所有病毒都能用疫苗來(lái)抗擊。某些類型的病毒不斷在變異或改變，結(jié)果就不可能開(kāi)發(fā)出針對(duì)所有變異的疫苗。

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