Health workers moved swiftly last week to bottle up the outbreak of the deadly Ebola virus in Zaire. But students of epidemics warn: Wait for the virus next time.
As farmers clear jungles, as tourists trek into exotic locales, as airlines and highways stitch continents more snugly together -- microbes quarantined by nature for millions of years threaten to spill across the globe.
Ebola was one.
But dozens of others have appeared in the past 30 years, including the microorganisms that cause AIDS, Lassa fever, Lyme disease, Legionaire's disease, Rift Valley fever, Brazilian purpuric fever, Venezuelan hemorrhagic fever and hanta virus pulmonary syndrome.
What are the chances that a so-called "emerging virus," or one of several resurgent diseases, will erupt in a global epidemic to rival AIDS?
No easy answer
"There is no clear-cut answer to that question," said Dr. Phillip K. Russell, retired commander of the U.S. Army Medical Research and Development Command, and now professor of international health at the Johns Hopkins School of Hygiene and Public Health.
"We have seen new strains of influenza arise by genetic recombination, presumably from the animal influenza virus, that have caused global pandemics," he said, citing the 1918-1919 influenza outbreak that killed at least 20 million people worldwide.
"We have seen the HIV [human immunodeficiency virus, which causes AIDS] emerge from an African primate source" and cause a pandemic "that is going to involve tens of millions if not hundreds of millions before this is all over," he said.
"The presumption is that there is a lot of potential out there for this to happen again, Dr. Russell said.
Arno Karlen, the author of the recently published "Man and Microbes," said that through history, as humans migrated, irrigated fields, built cities, explored continents or changed their sexual practices, they unwittingly opened the door for an onslaught of diseases.
'Progress' brings disease
The development of wool clothing provided a snug habitat for lice, spreading typhus. Air conditioning and whirlpool spas can incubate Legionaire's disease. Irrigation ditches nurture snails that carry schistosomiasis -- one of three diseases that annually take the greatest toll in human lives.
The Spanish conquistadors brought smallpox to the Americas, decimating the Aztec and other civilizations.
Advances in navigation and ship construction in the early 19th century expanded world trade, but unleashed cholera from the Bengal region of India, where it may have quietly existed for centuries.
"It could come and kill in a day or in hours," Mr. Karlen said. "It produced terror."
Now, air travel has placed every spot on the globe within a few days' travel of every other spot. That has helped propel the AIDS epidemic, which threatens to become the deadliest in history.
Newly discovered diseases aren't the only threat. Rapidly mutating viruses can assume new and more virulent forms. Diseases that had all but disappeared, such as tuberculosis and yellow fever, have re-erupted after health officials stopped worrying about them.
"It isn't as though the United States is immune to these diseases that we consider Third World diseases," said Dr. Diane E. Griffin, a professor at the Johns Hopkins School of Medicine.
"It's like tuberculosis -- you quit paying attention because you've thought you've accomplished everything. And before you know it, you're in trouble again."
As of now, Ebola isn't likely to threaten large numbers of people outside Africa.
While it is one of the deadliest known human diseases -- killing up to 90 percent of the people it infects -- it is not easily transmitted.
"It apparently takes a fairly high dose [of viral material] or direct contact with a victim's blood or with a contaminated needle," Dr. Russell said.
The concern is that an Ebola strain might appear that could be passed on as easily as the common cold. There is precedent for this.
"Viruses have a tremendous potential to mutate and change," Dr. Russell said.
"Most of the changes are detrimental to the virus. But sometimes changes occur that are advantageous and increase its ability to cause epidemics.
"HIV is a classic case. It's changing all the time. And we've seen change its genetic pattern that apparently increased its transmissibility."
Greater threat than Ebola
Researchers see a more immediate threat than Ebola, at least to the United States.
It is the potential re-emergence of a disease that had all but disappeared from North America: dengue fever, also known as "break bone fever."
In its most familiar form, the mosquito-borne dengue virus produces a painful but not deadly illness. But a new strain has emerged in Southeast Asia and elsewhere that triggers internal bleeding.
It can be life-threatening. There is no treatment.
"There was a huge outbreak of dengue in Cuba a number of years back," said Dr. Gregory Gurri Glass, associate professor of immunology at Hopkins' public health school. Now all four strains of the illness, including the most severe form, have spread through the Caribbean.
"We may well be looking at the potential for things to develop all over the Americas," Dr. Gurri Glass said.
U.S. not prepared
Both the United States and the world are not prepared to quickly spot and track a fledgling epidemic, specialists say.
"What needs to be appreciated is that we don't know much about what is going on out there," Dr. Gurri Glass said.
"There is not much active surveillance for infectious diseases in this country."
Dr. Griffin noted, "There is no global system for tracking these virus outbreaks.
"Those systems were in better shape two or three decades ago. They were pretty much dismantled."