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HE GOOD NEWS IS that many viral ailments, like measles, mumps, polio, hepatitis B, rubella and rabies, can be prevented by immunizations. (There are other vaccines to prevent bacterial diseases like diphtheria, whooping cough and pneumococcal pneumonia.) Influenza, too, can be prevented with immunizations. Now, at the outset of the flu season, is when the federal Centers for Disease Control in Atlanta recommends vaccinations for high-risk people - those over 65 and those suffering from chronic illness. According to the Centers for Disease Control, vaccination of those who are in this category ''is the single most important influenza-control measure.''

The first influenza vaccines were developed in the 1940's, after the isolation of the two main types of influenza: A in 1933 and B in 1940. (Influenza Type A is more likely to result in complications.) The modern versions of flu vaccines are derived from non-living, highly purified viruses of the specific types of influenza that have been circulating most recently.

For this winter, epidemiologists at the Centers for Disease Control recommend two flu shots: a basic one to protect against three strains of influenza A and B, and another to protect against a new variant. This new strain has caused outbreaks in several Asian countries and was detected too late to be included in the preparation of the basic vaccine.

A remarkable characteristic of the influenza virus is its ability to change antigens every so often, thus outwitting the ability of the human immune system to produce antibodies. For this reason, humans may suffer repeated attacks.

F DOCTORS TOOK specimens from all patients with infections and sent them to laboratories for identification, technicians would be overwhelmed. Even if laboratories could cope with such a workload, virologists still might not be able to identify the bug. One reason is that viruses cannot be isolated from all samples. Another is that scientists have not developed the techniques to identify the many viruses that are believed to be the cause of several ailments.

Occasionally, however, laboratory testing pays off. In 1975, after their children had suffered a series of similar symptoms - rash, body aches and fever - several mothers in Lyme, Conn., insisted that doctors investigate. They discovered it was a form of arthritis that was caused by ticks and is potentially curable by antibiotics. The bug was named Lyme disease and is now known to occur throughout the world.

Diseases are sometimes not identified until a dramatic outbreak makes detection possible. Before the summer of 1976, many cases of pneumonia were assumed to be caused by viruses. But that summer, members of an American Legion group attending a convention in Philadelphia contracted a mysterious strain of pneumonia. After studying the group, doctors were able to identify the ailment, now called Legionnaire's disease, as a bacterial infection. Doctors now know that 10 percent of cases previously called viral pneumonia were Legionnaire's disease.

Scientists first identified viruses in the late 1800's, when they found that some microorganisms passed through tiny filters that successfully screened out bacteria. One of the first such agents, found in 1902, was the virus that caused foot-and-mouth disease.

It was not until this century, with the invention of the electron microscope, that viruses could be seen. But most detection became possible when techniques were developed to grow viruses in the laboratory. The earliest attempts involved plants and animals with limited results. Greater success came in the early 1930's, when scientists discovered more fertile mediums for growing viruses - mice and eggs. Another breakthrough occured in the late 1940's, when scientists learned that viruses could be grown in cells in test tubes, a step that allowed a wider range of experimentation.

When health officials began keeping track of vital statistics a few centuries ago, they saw that certain ailments strike more often during one season than another. Those observations have been recorded more extensively in recent years with expanded medical surveillance. So it is now known, for instance, that influenza strikes most frequently in the cooler months - from October through April in the Northern Hemisphere and from May through September in the Southern Hemisphere.

Why should one virus appear seasonally and another have no discernible pattern? Scientists know of no changes that occur in viruses to account for seasonal patterns. Thus they seek such answers in environmental factors and in the natural habitats -called reservoirs - of viruses. Rabies, for example, is a disease of animals that affects humans only through animal bites. Thus, many cases of rabies can be traced to increased outdoor exposure during the spring, summer and fall. Doctors know that only humans are susceptible to polio and measles, but have not identified the reservoir for most other viruses.

Virologists have yet to discover where viruses hide between outbreaks. The reservoir for human influenza infections, for example, is man. But some virologists suspect that new strains develop and lie dormant in pigs, horses and birds.

OCTORS DO NOT know why a bug goes around when it does, or what causes an outbreak of viral illness at the time it strikes. Another unanswered question is why some viruses leave without a trace, while others remain dormant and then flare up years later, like chicken pox, which can recur as shingles, a painful nerve inflammation.

With modern techniques, studies have shown that many viruses can be regularly found in healthy people, called carriers. Some viruses may attack just one or two people sporadically, or they may sweep through the community. Why a virus will attack just a few people or suddenly explode to cause an epidemic is another mystery.

Doctors have eradicated only one viral disease -smallpox - and the prospect of achieving similar success with others is limited. Scientists have developed vaccines for previously unpreventable infections. Advances have been and no doubt will continue to be made in identifying and treating the many illnesses that exist. But chances are, no matter what progress medical science will bring, there will always be a bug going around.