In an article Sunday, Kenneth Sell was identified incorrectly as scientific director of the "CDC allergy institute." He is with the National Institute of Allergy and Infectious Diseases.

Row upon row of freezers in the basement morgue of a red-brick building here holds a quarter of a million vials, each containing blood from a victim of an epidemic of an infectious disease.

The frozen collection at the national Centers for Disease Control (CDC) stands as stark refutation of the promise of only a generation ago: that advances in science would soon make epidemics a thing of the past.

Once, after the conquest of diphtheria, smallpox and polio, Americans believed that modern medicine had beaten infectious disease. More antibiotics and vaccines were coming, and more success was thought to be imminent.

But today, new epidemics are raging, one upon another, of diseases most people cannot even name. Each year, the CDC investigates more than 1,000 new outbreaks of diseases such as Acquired Immune Deficiency Syndrome (AIDS), Legionnaire's Disease and toxic shock syndrome.

The story of these diseases is the ancient one of evolution. An opportunistic bug seizes on a slight environmental change.

Medical science's failure to conquer infectious disease should have been expected. Each human generation is 20 years, but, among bacteria, a generation can pass in half an hour. Bacterial civilizations can rise and fall in weeks.

Technological advances as diverse as air conditioning and tampons have created ideal breeding grounds for germs that had been around for millennia. Modern transportation has conveyed exotic germs into new populations. The life style of some homosexuals seems mysteriously related to an epidemic of AIDS, one tragic new disease. Even hospitals are the cause of new deaths.

"Organisms have found new ways of getting to us, mostly through man's own advances," said Dr. John Bennett, assistant director of CDC's Center for Infectious Diseases.

AIDS appeared suddenly in the United States in 1981. Homosexuals in widely scattered cities were struck. First called the gay plague, it then appeared among drug addicts, Haitians, hemophiliacs and finally children.

It wipes out the body's immune system. Of the 1,128 persons reported suffering from AIDS since 1981, none has completely recovered and 423 have died.

Although its cause is unknown, AIDS appears to take advantage of changes in life style and technology. It has certainly been "amplified," Bennett said, by the intense sexual activity of "fast-lane" homosexuals who frequent bathhouses and bars in large cities.

Drug users' sharing of needles may also transmit the disease. And AIDS may be spreading through blood banks to hemophiliacs.

Toxic shock syndrome is an example of a microbe that adapted itself to a new product designed specifically for cleanliness and convenience. The improved tampon, with its increased absorbency, appears to provide a hospitable home for an old kind of infection, familiar staphylococcus germs.

The disease can strike anyone--living in wounds, infecting burns, hiding under bandages--but its most frequent victims are young women between the ages of 15 and 24 who are using tampons.

Legionnaire's Disease came to light as a puzzling pneumonia outbreak among American Legion conventioneers in 1976. Legionella had lived harmlessly in the soil and water for thousands of years. But it has found a hospitable new home in modern buildings' plumbing and air-conditioning systems.

Hospital-caused infections are almost classic proof that in the battle between man and microbes, the bugs have the evolutionary edge.

Great teaching hospitals have been built and in them techniques developed so that the very ill are saved with miraculous skill and modern technology. For the most part, every new device helps heal, but with each comes a new danger of infection. Antibiotics have been used until some microbes are now resistant not to a single drug, but to dozens.

New diseases are being imported that were once isolated beyond U.S. borders. "Modern transportation has made the entire world a global community. We don't have the insulation we once had," CDC's director, Dr. William Foege, said. "Everything is just down the street when it comes to health."

rom ancient times to the 20th century, infectious diseases often shaped history.

"The really dramatic cases occurred when civilized peoples with complex immunity to disease came into touch with previously isolated peoples . . . . This happened over and over again," University of Chicago scholar William H. McNeill said.

The Black Death of the mid-1300s killed one-fourth to one-third of the population of western Europe and Russia. Plague had lived for centuries in colonies of rats in Asia, moved west with caravans on the Silk Route and was apparently spread from the Crimea to medieval European ports by Genovese sailors. The plague disrupted the European continent for centuries, changing economies, religion, art.

"Everything you can think of was touched by the plague," McNeill said.

Soldiers of every war once suffered more serious losses from disease than warfare. At the turn of the century, British deaths from disease in the Boer War were five times higher than combat casualties.

The late Rene Dubos, an American microbiologist and author, once noted that disease contributed greatly to the European conquest of America, as smallpox, measles, scarlet fever, tuberculosis and other infectious diseases "decimated the Indians and contributed to the breakdown of their physical stamina."

Scholars now believe that the small band of Spaniards who conquered the Aztec Empire and ruled most of the New World for centuries could not have done so if they had not introduced smallpox into Mexico. Hernando Cortez's men were immune to the disease, then raging in Europe, but the Aztecs had no immunity, and millions died.

Improved sanitation and public health measures in the 18th and 19th centuries held the promise of eradicating many infectious diseases spread by rats, contaminated water and untreated sewage.

Then came sulfa drugs in this century. Penicillin was developed from the accidental growth of mold in a laboratory, and it killed bacteria. Jonas Salk later developed the vaccine for polio.

Medicine seemed on the edge of an era when no disease would be beyond the reach of curative or preventive drugs.

Foege, 47, cites his family history to demonstrate the dramatic change. Five of his grandmother's 10 children died as infants. But Foege and all five of his siblings survived to adulthood (with Foege now helping to lead the world battle against smallpox).

An American baby born in 1900 was expected to live an average of only 47 years, but an infant born today is likely to live to age 73. Foege calls control of infectious childhood killers a "one-generation miracle." But he warned that the government must maintain "what we've achieved."

"It's like a clearing in the jungle," he said. "Just as soon as you stop cutting, it overgrows."

Even today, said National Institute of Allergy and Infectious Diseases Director Richard M. Krause, "what is often forgotten is that despite all the advances that were made, nearly 25 percent of patients who seek medical care do so because of infection."

Infections are estimated to cost $24 billion annually in medical bills and absenteeism from school and work in this country, as well as a great deal of misery.

"After giving antibiotics and vaccines their due," Krause said, "the fact is that either directly or indirectly infections remain one of the common causes of death."

Illnesses such as influenza and its complications, primarily pneumonia, are still a major threat to the elderly and those with chronic diseases. Although seldom recognized as such so menacing, together they rank sixth among the nation's top 10 killers.

If all infectious diseases were lumped together, as cancer and heart diseases commonly are, they would become the country's No. 4 killer, CDC's Bennett calculated.

Infectious diseases are still the No. 1 killer in developing countries.

he greatest global achievement in the war against infectious disease was the elimination of smallpox. The last case occurred in Merca, Somalia, in 1977 after a decade-long international effort led by the World Health Organization (WHO). But eradication of smallpox offers almost a textbook example of how evolution may circumvent the best efforts of 20th-century science.

"Once smallpox was eradicated," said Dr. Walter R. Dowdle, head of CDC's Center for Infectious Diseases, a curious disease called monkeypox "began cropping up in Zaire and the Ivory Coast. It was clinically identical to smallpox, but fortunately didn't seem to spread like smallpox."

When the more virulent smallpox virus was eliminated, its distant relative monkeypox had new opportunities for expansion. WHO experts and the CDC are carefully watching monkeypox, which kills about 15 percent of its victims.

Influenza is the best known case of a virus that changes so frequently that vaccines must continually be developed to keep up with it. Malaria-carrying mosquitoes also have become resistant to bug-fighting pesticides such as DDT; leprosy is becoming resistant to drugs used to treat it, and bacteria of several types have managed to retaliate against the potent antibiotic penicillin, producing their own enzymes to inactivate the drug.

An unusual form of attack--a piggyback bug--has also been detected, said Dr. Kenneth Sell, scientific director of the CDC allergy institute. A subviral particle that cannot survive on its own hitches itself to the hepatitis B virus to create a form of the liver disease far more virulent than normal. It not only causes a life-threatening, short-term illness but also may increase the chance of chronic infection and long-term risk of liver cancer.

First detected in Italy in 1977, it was named the Delta agent. Already the cause of an epidemic among Yucpa Indians in western Venezuela, it is showing up in high-risk groups in the United States, including intravenous-drug users and frequent recipients of blood transfusions.

Infectious organisms may also team with man-made drugs in ways no one understands. Reye's Syndrome in children causes sudden vomiting and fever that may lead to convulsions and death. This disease appears to be linked mysteriously to use of aspirin to treat children's influenza or chicken pox. In more than 20 percent of the estimated 1,200 U.S. cases each year, it results in death.MM icrobiology was about as scientific as voodoo until inM vention of the microscope in the 1600s allowed scientists such as Anton van Leeuwenhoek to view "very little animalcules" for the first time. Two hundred years later, Louis Pasteur and others proved that some "animalcules" caused disease.

The best-known disease-causing microorganisms are bacteria, primitive rodlike or spherical organisms one-hundred-thousandth of an inch long. They carry leprosy, tuberculosis, cholera, plague, venereal disease, "strep" and "staph" infections.

They are easy to see in ordinary microscopes and grow in the laboratory. From the 1930s on, progress in developing drugs to fight them was tremendous.

Viruses, the smallest known infectious agents, are more elusive. They could be tracked only indirectly by showing transmittal of diseases--including smallpox, yellow fever, polio, mumps, measles, and influenza--until the electron microscope's greater magnification penetrated their world. The influenza virus was discovered in 1933.

Because they must take over the genetic machinery of a living cell in order to reproduce, viruses are difficult to grow in the laboratory. But continuing improvements in laboratory study have meant a tremendous growth in the number of known viruses affecting man--from 65 in 1950 to more than 400, CDC's Dowdle said.

Fortunately, man lives amid countless microorganisms that largely do no harm. Some even help. But, given the changing nature of man's relationship with these microscopic creatures, allergy institute Director Krause warned against being "lulled into a state of complacency . . . . Infections are always going to be with us."

"I don't think the good Lord all of a sudden created a new group of microbes to plague man," said Dr. William Jordan, an infectious-disease expert. "It's the setting that changes."NEXT: Legionnaire's Disease