For most, it's just another risk of winter -- a few days of fever, aches and feeling rotten.

But for nearly 40 million Americans -- mainly the elderly and the chronically ill -- flu is a life-threatening disease. These people are vulnerable to influenza not because of the disease itself, but because of its complications.

In combination with pneumonia, flu is the fifth-leading cause of death in the United States. The worldwide flu epidemic of 1918 claimed an estimated 20 million lives, including about 500,000 Americans, more than four times the number killed in World War I.

Influenza, which got its name centuries ago when Italian astrologers thought illness was an "influence" from the stars, is caused by a fickle, fast-moving and highly contagious virus.

"In the age of jet travel, it can leapfrog across continents in a matter of a few hours," says Dr. Morgan Delaney, a lung specialist and associate professor of medicine at George Washington University Medical Center.

Even worse, the flu virus is actually a whole family of viral strains -- and they are constantly changing.

That makes flu a moving target for epidemiologists and other public health experts. No single vaccine offers protection against all flu viruses. Each year's vaccine is an educated guess based on recent patterns of worldwide flu activity.

By the time the first case of flu is confirmed -- any day now -- and the specific makeup of the virus is isolated and identified, it will be too late to adjust the 1985-86 vaccine. The formula for that vaccine was chosen last spring by the Centers for Disease Control and the Food and Drug Administration so that drug companies could grow enough virus to produce millions of doses of vaccine in time for the current season.

"Sometimes they're right, and sometimes they're wrong," says Delaney. "We won't really know until too late."

"We can't predict exactly what the virus will do," says CDC epidemiologist Nancy Arden. "We get as much information as we can from our monitoring around the world. It's very much like a tremendous puzzle."

Flu strains are divided into three main types: A, B and C. Type A strains usually are more severe than the others. Individual strains are named according to where they were first identified -- for example, A-Texas or B-USSR. More technically, flu viruses also are identified by the type of surface antigens -- hemagglutinin (H) and neuraminidase (N) -- that they contain. There are several types of each antigen. The A-Philippines virus, for example, is also known as the H3N2 virus.

This year's flu shots combine three vaccines, protecting against A-Philippines, A-Chile and B-USSR.

The A-Philippines virus dominated last year's flu season, accounting for about 97 percent of the viruses identified, says CDC's Arden. The B-USSR virus was responsible for most of the rest, while the A-Chile virus accounted for only a handful of cases.

CDC epidemiologists are betting that the same three viruses will dominate this year's flu. They hope that means another relatively light flu season, because many people have built up some immunity to those viruses through previous exposure.

The flu virus changes constantly, but major shifts occur about once a decade. These major shifts -- such as the Asian flu of 1957 and the Hong Kong flu of 1968 -- are devastating because they introduce a new viral strain against which the population has built up little or no immunity.

The three viruses expected to dominate this year are all variants of older flu strains: A-Philippines is a variant of the Hong Kong flu of 1968, A-Chile is a variant of a strain that disappeared in the mid-1950s and reappeared as the Russian flu in 1977, and the B-USSR showed up two years ago.

Although this year's vaccine is the same as last year's, the leftover supply cannot be used because it had an expiration date of June 30, 1985. People who received the vaccine last year still need to be revaccinated, because the effect wears off during the year.

Flu shots are recommended for health care workers and for people who fall into high-risk categories defined by an immunization advisory committee of the Public Health Service. These target groups include:

*Anyone with chronic heart or lung disease requiring hospitalization or regular checkups during the past year.

*Residents of nursing homes and other facilities for the chronically ill.

*Doctors, nurses and other health care workers who are often exposed to high-risk patients.

*Otherwise healthy people over 65.

*Adults and children with chronic illnesses -- such as diabetes, kidney disease, anemia or asthma -- severe enough to have required hospitalization or regular checkups during that past year.

The vaccine, considered 70 to 90 percent effective, usually is given in a single dose, but children who have never had a flu shot get two doses a month apart.

In past years, as few as 20 percent of those in the high-risk categories received flu vaccine. One reason immunization rates have lagged recently is residual fear from the ill-fated swine flu vaccination program of 1976-77, when more than 100 persons died of reactions to the vaccine itself and hundreds more suffered temporary or permanent side effects from a complication called Guillain-Barre syndrome.

"There is a great fear that that still represents a significant risk, which is not the case at all," says Delaney. There is less than a one-in-a-million risk of a serious reaction to the current vaccine, he says.

The only group specifically vulnerable to the vaccine itself is people who are allergic to eggs, because the flu viruses are grown in chicken eggs in the laboratory.

No matter what type of flu strikes, the range of symptoms is about the same: headache, fever, runny or stuffed nose, cough, sore throat, weakness, aches and pains and sometimes, nausea. Usually flu strikes suddenly and lasts about a week.

For otherwise healthy flu victims, the traditional treatment is rest, fluids and a mild painkiller such as aspirin (which also reduces fever) or acetaminophen. But children and teen-agers should not take aspirin for flu because of the suspected increased risk of contracting Reye's syndrome, a rare but potentially fatal neurological disease. Acetaminophen is considered safe for treating flu in children and adolescents.

Antibiotics don't work against flu, because they fight bacteria and the flu is a virus. The only approved anti-flu prescription drug is amantadine (brand name: Symmetrel), which has been shown to be 70 to 90 percent effective in reducing the severity and duration of type A flu symptoms if given within a day of their onset.

Amantadine is useful in controlling the spread and severity of flu outbreaks in high-risk groups.

"We don't think it's used nearly as much as it should be, particularly in institutions like nursing homes," says Dr. William Jordan, director of microbiology and infectious diseases at the National Institute of Allergy and Infectious Diseases.

But amantadine is not considered a substitute for vaccination, because it does not protect against type B flu. It also is more expensive than inoculation and causes side effects, such as insomnia and lightheadedness, in up to 10 percent of those taking it. A new drug, rimantadine, seems to have fewer side effects, Jordan says, but it is still in the experimental stage.