The threat, not well understood outside the medical community, has been rendered with authoritative starkness since news broke of the flu vaccine shortage. Lives are at risk. Every year, we are told, influenza kills multitudes of vulnerable people, and the death toll is often repeated: 36,000 die annually in the United States alone.

The lethal efficiency of the flu has come as a surprise to many, it turns out, because almost no one is ever officially classified as dying of influenza.

There is no public national accounting of who had influenza listed on their death certificate. But of the roughly 2,500 deaths estimated by city health officials to have been caused by the virus in New York City in 2002, influenza was listed as the cause of death for only two people. 

The previous year, not a single death certificate listed it, according to city health records.

It turns out that while federal and state health officials all proclaim great confidence in the annual estimates of those killed by the flu, the system for deriving the numbers is complex, imprecise and anonymous.

Part of the problem is that the flu is not often a direct killer but rather the fatal ingredient in a mix of factors that lead to someone's death. 

"It sets the stage for the susceptible host to crumble," said Dr. Jonathan M. Raskin, a pulmonologist in Manhattan. 

The system by which the Centers for Disease Control and Prevention in Atlanta and the New York City Department of Health and Mental Hygiene come up with an average annual death toll for flu are the same and are relatively simple.

Dr. Farzad Mostashari, an epidemiologist with the City Health Department, said that both the department and the C.D.C. look at the average number of pneumonia deaths during a given period and set them against what is known about flu activity during that period. When the flu is present, they check for marked increases in the number of pneumonia cases. All the cases above the average are attributed to the flu, he said.

New York City fits in with all the national patterns. For instance, the majority of flu fatalities occur among the elderly. In 2002, pneumonia/influenza did not make the list of the top 10 killers for people 35 to 54. However, for those 65 to 74 it ranked sixth, killing 290 people. For those 75 to 84, it ranked third, killing 747.

Dr. Mostashari also noted that studies have led researchers to believe that flu deaths have been coded as heart disease in many cases.

While he expressed confidence in the statistical analysis that leads health officials to come up with average annual deaths, he did say it was a best guess, given the variety of factors that can complicate the severity of the flu.

Doctors and health officials also noted that each year there have been wide fluctuations in the estimates of those killed by flu because different strains are more deadly. 

The virus changes every year, and the more it does, the more likely it will find victims who cannot resist it. These figures, again, are determined by the number of pneumonia deaths during flu season.

The strain the public will face this year is still unknown, though the few cases that have been reported indicate that it is a familiar one. There are surveillance centers across the city and the nation to provide officials with an analysis of flu strains as they begin to surface in communities; it is this data that is used to track flu activity.

A disaster like the Spanish flu pandemic, which struck in 1918 and killed some 600,000 people in the United States, was a very different situation. Even if there had been vaccines then, they would have had little effect because the flu strain was unknown. 

It is interesting to note that the victims of that virus were for the most part young and healthy, as opposed to the normal situation in which infants, the elderly and the chronically ill are most at risk.

This year, the notion of those most at risk has been drilled home as never before - children under 2, adults 65 and older and people with chronic health problems like asthma.

Dr. Stephen S. Morse, the director of the Center for Public Health Preparedness at the Mailman School of Public Health of Columbia University, explained why those groups are vulnerable.

"One of two things can happen," he said. "The flu can get in the lungs." In that case, it often leads to pneumonia. The second way the flu can prove fatal, Dr. Morse said, is that it can lead to secondary infections.

However, neither the city nor the federal government breaks down who dies from flu beyond tracking pneumonia cases. Given this season's vaccine shortage, that could make deciding who gets shots even more difficult if it turns out that there are not enough for the people in the high-risk categories.

The Centers for Disease Control has convened a panel of ethicists to look into just that question. Without data to suggest that someone with emphysema, for example, is more vulnerable than someone infected with H.I.V. or AIDS, any further rationing will have to be left up to private physicians.

As it stands, the vaccine shortage will likely leave doctors in the position of prescribing more anti-viral medication than in years past.

Dr. Raskin, the pulmonologist, said that he typically prescribe anti-viral drugs regardless of whether patients have had a flu shot. He noted that the public does not seem to realize that there are several anti-viral drugs that can be very effective in saving lives, provided a patient is diagnosed and treated quickly.

The doctor, who said he treats a large number of at-risk patients, added that he relied mainly on observation to determine if a person is suffering from the flu.
"Someone comes to me with a viral illness, there is a battery of tests we can run," he said. But that would cost upward of $5,000 and take weeks, he said, so he relies mainly on the symptoms they show.

A cold and a flu may often be confused in common conversation, but Dr. Raskin said there are some telltale signs of flu. Namely, those afflicted with it usually have a very high fever, 102.8 degrees or more, and are so sore they can barely move. "You just can't get off the bed," he said.

So far, he said, there has been no great demand for anti-viral medications.