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简单的生物体，但是对其他生物体有可怕的作用。

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

一种病毒的作用可以像普通感冒一样轻微，或像艾滋病病毒那样致命。

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

但是，所有病毒有一个共同的目的：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. 

病毒的结构是这样的：它能依附在细胞上，然后病毒的DNA攻击健康细胞，用它来制造更多的病毒。

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，宿主细胞激增并释放新的病毒细胞，依附在其他健康细胞上，再次开始这样的繁殖循环。

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为一场自然灾害。

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

在1918年，一场大面积的流感大流行袭击了美国每一个大城市。

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

科学家们已经开发疫苗来抗击一些有害的病毒。

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

疫苗通过用无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.

如果那种病毒再次进入人体，被称为记忆细胞的免疫系统一部分就能够识别和破坏它。

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来抗击。某些类型的病毒不断在变异或改变，结果就不可能开发出针对所有变异的疫苗。

责任编辑：admin