LONDON — London. -- "You might get some spread [of the Ebola virus] on a limited scale,'' said University of Chicago infectious-disease expert Dr. John Flaherty, ''but the scenario presented in movies like 'Outbreak,' where it just sweeps across the globe and annihilates everybody, is kind of far-fetched.''
That's typical of the comments by health authorities since the start of the panic over the outbreak of Ebola fever in Zairian cities. But just in case you're feeling relieved by all the official reassurances that it can't happen here -- don't be.
Four months ago, long before current panic, I made a special trip to talk to Dr. Alfred Crosby of the University of Texas, the world's leading authority on the history of disease.
Dr. Crosby's best-known work, ''Ecological Imperialism,'' was an epidemiological study of the European conquest of the New World. He showed that 90 percent of the work of killing the native peoples was done not by European muskets but by European microbes, and that European and African newcomers could never have become the dominant populations of the Americas otherwise.
The disease transfer was all one way because Europeans lived in densely peopled towns and cities where far more virulent diseases had evolved.
The New World peoples had no immunities to those basically urban diseases, and they had no comparably dangerous diseases to give back. So they died like flies -- 90 percent casualties within one generation of first contact was commonplace -- and the immigrants took their places.
But I wanted to talk to Dr. Crosby about Africa. Tropical Africa had few big cities either when the Europeans arrived -- but it was the Europeans, not the Africans, who died like flies. Why? And what relevance might it have for us now?
Oh, he said, that's easy. Africans always acquired the same immunities as Europeans after a while, because a land bridge connects Africa to Europe and Asia. And they had a whole lot of other diseases the Europeans were vulnerable to, simply because Africa carries the greatest ''disease burden'' in the world.
Indeed, he added, the strange fact that Africans, unlike the peoples of Europe and Asia, have spent most of their history in small, village-based societies may also be due to the continent's unique disease burden. It's hard to imagine something like the Han empire or the Roman empire springing up in an environment where large domesticated animals simply do not survive.
But why is Africa cursed with this especially great disease burden, I asked innocently. And that's when the conversation got really interesting. Also a bit frightening.
Diseases co-evolve with their hosts, and the process takes time. For human beings, Africa is the ancestral home. It is also an ideal environment for the evolution of new diseases.
''The wet tropics have the greatest number of species,'' Dr. Crosby explained; ''more quadrupeds, more birds, more micro-organisms. So if there are any nasties out there, most of them will be in the wet tropics.''
''A bigger chunk of the wet tropics is in Africa than anywhere else except for the New World, and people have lived in the New World for a much shorter period of time. So you have the most ancient of human populations in Africa, living among more species of micro-organisms than have been in a position to attack human beings in any other place in the world.
''And so that's probably where AIDS comes from, and there's something out there called Ebola fever which is very frightening indeed, there's something called Rift Valley fever, there's lassa fever. As the African population increases, as cash farming and exploitation of the environment increases, more and more Africans are pressing harder and harder on the environment.
''On the mountain-sides, you can see the population just creeping up like a noose toward the snows. More and more people are poking into every corner of this zoologically fascinating ecosystem -- so that's where most of the 'exciting' developments are going to happen.
''We haven't conquered nature, you know. We've just annoyed the hell out of her.''
So I asked Dr. Crosby whether it was imaginable that new diseases coming out of Africa -- coming very fast these days, due to the speed of international travel -- could have the kind of impact on the current global population that European diseases had on the native people of the Western Hemisphere.
''Yeah,'' he replied, and then paused to contemplate the enormity of what he had just said.
''I don't want that answer, but the answer is yes. We're all right so long as the disease is slow-moving. . . . Thank God AIDS is a venereal disease. Venereal diseases move slowly. But that is simply an accident. It could have been a breath-borne disease like influenza.
''When a new strain of influenza comes along, the medical community scrambles very fast to come up with a vaccine, get the vaccine into people's arms -- and we still all get influenza, because that's a process that moves more slowly than flu moves. Now if we get a fast killer like Ebola fever, and if it mutates just a hair and becomes transmissible by breath, then we have got the unimaginable on our hands.
''Nature will have solved her human problem by knocking us back to numbers that don't do such damage.
''Some strains of Ebola -- well, they usually knock off 50 to 90 percent of people infected, and there's no cure, no vaccine. This sounds science-fictiony, but it's a science-fictiony world.''
This was months before the outbreak of Ebola fever in Kikwit and other Zairian towns, but we're probably all right for the moment: This virus hasn't mutated. Dr. Crosby isn't a prophet -- yet.
Gwynne Dyer syndicates a column on world affairs.