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The next epidemic may begin with a dream. Or maybe a gunshot.

Whatever drives people and their livestock to cities - warfare, poverty, a vision of something better - also adds another pinch to the unhealthy soup of viruses and diseases that could brew the next killer outbreak.Urban slums and shantytowns, the "microbe magnets" of the world, are the grim arenas where health workers are trying to corral a host of mysterious new diseases such as Ebola and old nemeses such as cholera and tuberculosis in new and more virulent forms.

"It's all very frightening," said Dr. Michel Thuriaux, an expert in emerging diseases at the World Health Organization.

At a U.N. conference on urban problems, the messages of Thuri-aux and other health experts reveal their frustration in chasing and battling outbreaks. They understand where the enemy thrives but are often overpowered by its strength or baffled by its strategy.

Many of the problems are linked to the population flooding into cities, particularly in developing countries.

In the next 20 years, the United Nations predicts nine of the top 10 megacities will be in developing nations, including some in an epidemic-prone belt running across Africa and Asia. Three beleaguered centers alone - Lagos, Nigeria; Bombay, India; and Dhaka, Bangladesh - are expected to have a total of more than 70 million people in 2015.

Squalid and unsanitary conditions create perfect breeding grounds for mosquitoes that carry malaria, dengue fever and other maladies.

Overcrowding allows diseases to race through cities, where many people are already weakened from diarrhea and other ailments linked to fouled drinking water.

And the tendencies of villagers to bring their animals to the city create an opportunity for cross-species transmission of diseases - which some researchers believe could be the root of the virus that causes AIDS.

"They are bringing with them their rural ecology," Laurie Garrett, author of "The Coming Plague," said Friday at a forum on urban diseases. "They bring their pigs. They bring their goats. They bring their chickens. They bring their cows.

"It's fine to coexist with these other species in a dispersed rural setting, but when we come into the kind of contact in the cities . .. what you create is the ideal circumstance for ecological mixing of microbes."

At least 30 previously unknowndiseases have been identified since 1973, including the Ebola virus, a contagious hemorrhagic fever that surfaced in 1977 and re-emerged to kill 245 people in Zaire a year ago.

Old scourges also have roared back in changed or drug-resistant forms or have been reintroduced into areas by increasing international air travel. For example, a new strain of cholera appeared in southeastern India in 1992 and has now spread to Southeast Asia. A powerful form of tuberculosis has emerged among homeless people in the United States, WHO, which is based in Geneva, says.

"I wish I could say we had a magic bullet," Thuriaux said . "We don't."

What they do have now is speed. In October, WHO created a new project to send teams within 24 hours of an outbreak. In December, such teams went to Ivory Coast and Gabon a day after cases of Ebola were reported.

"Sometimes every hour counts," said David Heymann, director of WHO's emerging disease control and surveillance group.

An outbreak of cholera across South America in 1991 is now a textbook case in the globe-trotting nature of diseases.

It started with a merchant ship off Lima, Peru, pumping out bilge water from Southeast Asia that was contaminated with cholera bacteria. A chain-reaction began. Algae grew in the water. Shellfish fed on the algae and urban Peruvians who ate the shellfish came down with cholera, which causes severe diarrhea and can kill within hours.

Carried by travelers, the cholera spread west across South America - where the disease had been largely eliminated. Nearly 11,000 people have died since the first outbreak.

"The answer," said Thuriaux, "isn't to pretend it doesn't happen and hope it goes away."

His solution mixes pessimism with pragmatism.

"Countries have to learn to live with these frightening things just like a person would have to learn to live with a disability," he said.

"It's not an either-or situation: Either we're all perfectly healthy or we'll all die of Ebola virus. We'll probably have to live continually with Ebola. You can't run away."