病毒为何致命

In April 2009, the world was sent into panic by a mysterious agent.

2009年四月，一种神秘物质让世界陷入了恐慌。

One with the power to kill.

A disease was sweeping through populations, caused by an organism invisible to all, but the most powerful instruments - a virus.

The world health organisation said the swine flu has now spread to nearly every country in the world.

For all our scientific advances, we still have a lot to learn.

We have no idea, basically, how viruses make a living, other than the fact that they have to infect something and replicate.

Why do viruses exist?

We don't really know the answer to that question. We don't even know how many there are. There could be thousands of other viruses out there and we have no idea what they are, how they spread, what populations.

And because we struggle to understand them, they cause mass terror. (People must not panic.) But the carnage that was expected hasn't yet materialised. (England's chief medical officer, Sir Liam Donaldson, says the swine flu pandemic is considerably less lethal than had been feared.)

Once more, we appear to have got the calculations wrong. So when, and why do viruses kill?

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As events spiralled, so too did the rumour and fear. 随着病例的增加，谣言和恐惧也迅速蔓延。

The nightmare scenario everyone feared hasn't materialised. 人们担心的噩梦般的情景并未发生。

What this episode has told us is how difficult it is to predict how viruses behave. These tiny organisms continue to challenge even those that spend their careers studying them.

The virus is probably the smallest living organism. Viruses come in all different shapes and sizes. 病毒有各种不同的形状和大小。