They reproduce in a fanatical manner, then burst out in search of more cells to invade. 它们以疯狂的形式繁殖，接着拼命寻找更多的细胞作为入侵对象。

Not being living organisms themselves, they can afford to be very simple. 由于它们体身不是生物，所以它们可以保持非常简单的形态。

Many, including HIV, have ten genes or fewer, whereas even the simplest bacteria require several thousand. 包括人体免疫缺陷病毒在内的许多病毒只有10个可更少的基因，而连最简单的细菌也要有几千个。

They are also very tiny, much too small to be seen with a conventional microscope. 它们还小不可言，用普通的显微镜根本看不到。

It wasn't until 1943 and the invention of the electron microscope that science got its first look at them. 直到1943年发明了电子显微镜，科学家才首次见到了它们。

But they can do immense damage. 但是，它们可以起巨大的破坏作用。

Smallpox in the twentieth century alone killed an estimated 300 million people. 据估计，20世纪光死于天花的人就达3亿。

They also have an unnerving capacity to burst upon the world in some new and startling form and then to vanish again as quickly as they came. 病毒还具有一种令人吃惊的本事，能以某种新的形式突然在世界上出现，然后像很快出现那样再次很快消失。

In 1916, in one such case, people in Europe and America began to come down with a strange sleeping sickness, which became known as encephalitis lethargica. 举个有关的例子，1916年，欧洲和美洲有些人开始患上种古怪的昏睡病，后来被称之为“昏睡性脑炎”。

Victims would go to sleep and not wake up. 病人睡过去，自己醒不过来。

They could be roused without great difficulty to take food or go to the lavatory, 他们很容易被唤醒，起来进食或上厕所，

and would answer questions sensibly—they knew who and where they were—though their manner was always apathetic. 还能理智地回答问题——他们知道自己是谁，在什么地方，虽然他们的样子总是很漠然。

However, the moment they were permitted to rest, they would sink at once back into deepest slumber and remain in that state for as long as they were left. 然而，一旦你让他们去休息，他们便马上会再次陷入昏睡，长时间保持那种状态。