After decades of warnings about the inevitability of another pandemic of influenza, it is astonishing that health officials have failed to make clear to the public, even to many colleagues, what they mean by the word pandemic.

Generations of people have used the term to describe widespread epidemics of influenza, cholera and other diseases. But as the new H1N1 swine influenza virus spreads from continent to continent, it is clear that a useful definition is far more complicated and elusive than officials had thought.

And what is at stake is far more than an exercise in semantics. A clear understanding of the term is central to the World Health Organization’s six-level staging system for declaring a pandemic, which in turn informs countries when to set their control efforts in motion.

Dictionaries and medical journals offer little guidance. Their definitions can be too vague or too narrow, contradictory and clouded by jargon.

"There is a lot of misinformation in the medical literature, and it is really quite hard to figure out what is and what is not a pandemic,” said Dr. David M. Morens, an epidemiologist at the National Institute of Allergy and Infectious Diseases who has been studying the history of pandemics.

The word implies the rapid spread of an infectious disease to many countries in different regions, hitting each with more or less the same severity. But in fact, severity varies — not all people are infected at the same time, and not every country need be affected.

And there can be many other factors, including the numbers and percentages of people falling ill and dying; a population’s vulnerability to the disease, based on previous rates of infection; and the quality of health care facilities and disease monitoring systems.

Not least is that scientists do not know precisely how pandemics arise, what fuels them, why they vary in their lethality, why some occur in waves and why they stop. Health officials have long preached that with influenza, the only sure bet is to expect the unexpected.

The new swine influenza virus, which appeared suddenly after years of warning about a potential pandemic of avian influenza, upset the W.H.O.’s assumptions that most people have the same understanding of the word pandemic.

For years, the organization’s Web site defined an influenza pandemic as causing "enormous numbers of deaths and illness.” But the agency recently pulled the definition, apologizing for causing confusion and anxiety. One of the biggest problems in public health is communicating risk assessment.

United States and W.H.O. officials say their preparedness plans are intended for governments, not people in the street. Officials bristle at criticism that their messages and plans have led the public to equate the word pandemic with the Spanish influenza of 1918-19, the worst recorded pandemic in history, killing 20 million to 100 million people.

In preparing for the worst, officials have considered milder pandemics, said Dr. Nancy J. Cox, chief of the influenza division at the Centers for Disease Control and Prevention in Atlanta.

But Dr. William Schaffner, the chairman of preventive medicine at Vanderbilt University, said that "we, the public health community, deserve to be chided” about the confusion.

"We ought to be able to do a better job in communicating in an understandable way,” he said in an interview.

Scientists like to assert that theirs is an exact discipline. But like the terms "evidence -based medicine” and "peer review,” pandemic turns out to be another example of imprecise vocabulary that doctors use every day, assuming everyone understands their meaning.

Journals, textbooks and reference works use pandemic in discussing certain diseases, but rarely define the word. For example, the definition section of the Control of Communicable Diseases Manual, a standard reference work, includes "endemic” (said of a disease that is usually present in an area or a population group) and "epidemic” (more cases of an illness than would normally be expected) but not "pandemic.”

The disease manual’s editor, Dr. David L. Heymann, a retired assistant director-general of the W.H.O., said the term had not caused confusion in the past, but assured me in an interview that "pandemic will be defined in the next edition.”

Even the indexes of most major medical textbooks do not list pandemic. One is Harrison’s Principles of Internal Medicine, of which Dr. Anthony S. Fauci, who directs the National Institute of Allergy and Infectious Diseases, is a main editor.

"It’s a mistake, and I’m surprised it’s not there because it should have been,” Dr. Fauci said in an interview.

Government agencies do not have official lists of pandemics. Textbooks cite many recent and old ones, including these:

- AIDS. Many experts have called H.I.V. a pandemic. Others disagree, saying the virus is pandemic only in Africa.

- Cholera. Since 1817, most experts agree, the world has had seven pandemics of this bacterial illness, which causes severe diarrhea and dehydration.

Acute hemorrhagic conjunctivitis. Beginning in 1969, an enterovirus has caused tens of millions of cases of a highly contagious, acute, painful, but rarely blinding, form of hemorrhagic eye inflammation.

- Dengue. Since World War II, this mosquito-borne viral disease has spread widely in Asia and Latin America.

- Syphilis. A pandemic of the bacterial disease raced through Europe and Asia after Columbus’s return from America and during mass movements of armies in Europe.

Although pandemics have been classically limited to infectious diseases, the term has spread to noninfectious, chronic ones. For example, many health officials now speak of pandemics of obesity and heart disease.

Knowledge about past pandemics is necessarily incomplete; historical accounts cannot make up for the absence of modern disease monitoring and laboratory tests.

About 14 pandemics of influenza have been described since the 16th century, with the first indisputable one occurring in 1889.

In 1580, an influenza pandemic swept through Asia into Europe within six weeks, and at least 10 percent of Rome’s 81,000 residents died in the first week, said Dr. Michael T. Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota. Some Spanish cities were almost totally depopulated.

Dr. Morens, of the infectious diseases institute, said his studies of influenza pandemics left a confusing track record and "are rewiring our brains about thinking about influenza.”

"The medical literature will tell you there were three pandemics in the 1830s,” he said — "one from 1830 to 1832, a second in 1833 to 1834 and a third in 1836 to 1837. But I am beginning to think they were all one pandemic.”

Dr. Morens said he was puzzled as to why no influenza pandemics were recorded for nearly 150 years after the one in 1580, although there were some severe localized epidemics.

"A period of pandemic stability makes us wonder whether a pandemic comes at any time by chance,” he said, "or whether something about epidemic situations prevents pandemics,” or at least delays them.

The W.H.O.’s staging system has long been part of its plan for an influenza pandemic. Deep concern about a potential pandemic of the H5N1 avian influenza virus led the organization to convene a large meeting of experts in 2005.

Among other things, the experts recommended simplifying the staging system. A number of doctors ask why health agencies do not declare seasonal influenza a pandemic when it spreads around the world.

But Dr. Osterholm, the Minnesota expert, said that "you can’t use the terminology for just worldwide transmission, because if you did that, you would say every seasonal flu year is a pandemic.”

"To me,” he continued, "a pandemic is basically a new or novel agent emerging with worldwide transmission.”

Dr. Keiji Fukuda, an influenza expert who is an assistant director-general at the W.H.O., said in an interview that "as difficult as things are right now,” the problem of defining a pandemic and communicating risk "would be magnitudes worse and more confusing” if the agency had not dealt with AIDS, SARS and avian influenza.

Those experiences  prompted new international health regulations and pandemic plans, and allowed critical scientific information to be disseminated quickly, he said.

The process was "painful, sure,” he said. "But you can’t really do anything like this without having some amount of pain.”

New York Times