In the final part of a series of reports for the BBC's Newsnight, the award winning world affairs editor, John Simpson, looks at disease, one of the four evils which threaten global stability.

From Amazonia to the Antarctic, there is effectively nowhere on the planet we cannot reach within 24 hours.

The last of our horsemen, the spread of disease, has taken huge advantage of this.

Diseases which were once confined to the remotest places are now breaking out into the wider world.

The forests of West Africa and the Congo River have always contained an extraordinary variety of extremely virulent diseases. However, because few people ventured there, they did not spread.

Diseases can travel

As the forests are increasingly being invaded and cut down, the diseases are able to escape.

Aids may have come from these dark forests, and the Ebola virus appeared from there a few years ago.

Ebola is terrifying. Within days the body begins to haemorrhage from every orifice, and a horrible death usually follows.

It is not incurable. In a good hospital, with careful nursing, it can be treated. In Africa, these conditions rarely exist.

I travelled with the Newsnight team to the Tai forest in The Ivory Coast where Ebola has been isolated.

Scientists there, searching for the carrier of the disease to discover how it makes its way to the human population, trap every type of animal in the forest, which is relatively small.

If they can test 100 examples of each type, they should be able to trace the carrier.

It is no easy matter. There are dozens of different types of bat, monkey, rat and snake in Tai, and the search will take a long time.

Likely source

The scientists there have made an educated guess at the likely route the disease takes.

They think it is probably carried by a particular type of bat, whose excreta infect the monkeys in the forest.

The local hunters regard the monkey as a valuable delicacy, and if they find a dead one in the forest they are inclined to sell it for its meat.

If the meat is not properly cooked, Ebola can pass to the human population - with deadly effect.

The spread of Ebola is relatively difficult, since it is spread by bodily secretions.

Nevertheless if the virus were to ally itself with one which spreads more easily - measles, for example - the results could be catastrophic.

It would be perfectly possible, under modern conditions, for someone to catch Ebola in the Tai forest, carry it to Abidjan, the capital of The Ivory Coast, and then take it on to Paris or London or New York, where it would reach its most virulent stage.

Yet Ebola and other exotic viruses from the forests are unlikely to turn into the great plagues of the 21st century.

Greatest threat

What is far more likely is that the 20th century's worst disease, influenza, will make a comeback during the next few decades.

Fifty million people died in the 1918 outbreak. The World Health Organisation believes that a similar strain is certain to break out at some stage soon.

Two years ago a particular influenza virus leaped from poultry to humans in Hong Kong and South China.

In Hong Kong, which is small and well-administered, it proved possible to kill every chicken in the territory and prevent any further infection of human beings.

It was a close call. If the disease had escaped, the planes which take off from Hong Kong by the dozen every hour would carry the new strain of influenza to every part of the globe within a day.

The four horsemen of the new Apocalypse use our own systems against us precisely as a virus does.

Disease and crime use the possibilities of swift movement to spread everywhere.

War takes advantage of the information revolution to spread its cause and its message.

We shall soon see the effect of this in Chechnya, after the Russians have taken Grozny; terrorism and infiltration will bring new forms of conflict to the Russian heartland.

Above all, the horseman of environmental destruction uses the smallness of our world against us.