As viruses go, Sars is pretty pathetic. It’s hard to catch and not very powerful. Despite the horror stories, 90% of those who become infected go on to make a full recovery. On balance, then, it’s probably sensible for schools in Britain to stay open and for aeroplanes to carry on circling the globe.

However, what if it were Ebola? Since this filovirus was first identified in 1976 it has become a bit of a joke. Reports at the time said it dissolved fat and lots of Hurley/Posh surgically enhanced women thought it might be a fun alternative to liposuction. I’m just as bad. Every time I go to the doctor I always tell him I’ve caught Ebola just for a laugh.

Actually, it isn’t very funny. It attacks your immune system - but unlike HIV, which lets something else come along and kill you, Ebola keeps on going, charging through your body with the coldness of a shark and the ruthlessness of a Terminator.

First your blood begins to clot, clogging up your liver, kidney, lungs, brain, the lot. Then it goes for the collagen - the glue that holds your body together - so that your skin starts to fall off. Usually your tongue falls out, your eyes fill with blood and your internal organs liquefy before oozing out of your nose. Except for your stomach. You vomit that out of your mouth.

It’s not an exaggeration to say that Ebola eats you alive and then, to make sure you don’t die in vain, it finishes you off with a huge epileptic fit, splashing eight pints of massively infectious blood all over anyone within 20ft or so.

Nobody dies of Ebola with dignity and very few victims get better. Unlike Sars, the most virulent strain of Ebola, called Zaire, kills 90% of those who get it.

Now at this point you are probably thinking: so what? There is no Ebola in the world at the moment. Oh yes there is, but despite a 20-year, multi-million-dollar hunt nobody has been able to find where it lives. Some say the host is a bat, others say it’s a spider or a space alien. All we know is that occasionally, and for no obvious reason, someone comes out of the jungle with bleeding eyes and his stomach in a bag.

Tests have shown that the virus is simple and ancient. It has probably been hanging around since the days when Rio de Janeiro was joined at the hip to Cameroon. Over the years, therefore, it’s reasonable to assume that it has killed thousands of people.But because it kills so fast it could never travel. Now, though, with Zaire connected to the worldwide web of airline routes, an infected person could reach London or New York before he knew he was ill.

We saw this with Aids. Who knows how long this had been hanging around in the jungle, playing jiggy-jiggy with monkeys? When they paved the Kinshasa Highway that bisects Africa from east to west and the trucks started to flow, Aids burst into the world and, 25 years later, about 22m people were dead.

It may be that in years to come, when Aids has killed more people than the first and second world wars combined, historians will look upon the building of this road as the most significant event of the 20th century.

HIV, remember, is another pathetic virus. It can live for only 20 seconds in the air, it travels from person to person only if they engage in vigorous sex, and it takes 10 years to do to a person what Ebola manages to do in 10 days.

Sars has shown us just how devastating the jet engine can be as a carrier. A doctor gets poorly in a Hong Kong hotel and within weeks there are outbreaks all over the world. Even Canada got itself on the news.

Like HIV, Sars is also difficult to catch. Ebola is easy. In the 1990s scientists in America put an infected monkey in a cage on one side of a room and a healthy monkey in a cage on the other. Two weeks later the healthy monkey was dead.

Following a spate of Hollywood films, most people believe the human race is at greatest risk of annihilation from a giant meteorite or some kind of religious nuclear war. But if Ebola ever gets on a plane, experts say that 90% of us will be dead within six months. It is known in America, where they are good at names, as a "slate wiper".

This is why I’m slightly nervous about the world’s reaction to Sars. We like to think that governments have contingency plans for every conceivable disaster. But I got the impression over recent weeks that a lot of people have been sitting around in rooms saying "ooh" and "crikey" and "you can’t do that - think of the shareholders".

What we need is a scheme that would allow scientists and medical experts to impose, at a moment’s notice, a total ban on all flights and a global curfew. But who would run such a thing? The World Health Organisation doesn’t even have big enough teeth to take a bite out of that political colossus Canada.

The Americans? I fear not. Any disease that has a fondness for eating stomachs would head there first. Besides, if they couldn't find Osama for ten years what chance do they have of finding something so small that there could be a million on the full stop at the end of this sentence.

So, it’s the United Nations then. We’ve had it.