A child care worker in a large Army town mysteriously acquired hepatitis last summer. It was traced to the crowded day care center where she worked. Within three months, a hepatitis outbreak hit 46 more children and adults in the same center.

A child at another large day care center in the West got hemophilus influenzae meningitis, a disease that inflames the brain lining. In the following months, this usually uncommon disease spread to four more of the children, all under 2 1/2.

Five Fairfax mothers gathered one day a week to let their nine young children, all but one under 3, play together. One developed a diarrheal disease. Within four days all nine were sick.

A new public health problem is emerging in America: the spread of diseases among very young children in day care groups and centers.

"We're very sensitive about this whole issue," said a health officer who declined to be named. "We don't want to set off a panic. We're afraid people who oppose day care will blow this way out of proportion."

Day care centers, especially large, overcrowded ones that are poorly run, have become "a major hotbed" for spread of infection, according to Dr. John Bennett of the federal Centers for Disease Control.

The reason, said one state health official: "We're going through a dramatic change in care of our youngest children. We've been sending 4-and-5-year-olds to nurseries and day care for some time. Now we're sending 2-and-3-year-olds and infants. Until now it was rare to see five or six or a dozen toddlers together. Now it's the fastest growing part of day care." The result, suddenly, is the concentration of between 800,000 and 2 million children in diapers, non-toilet trained children who can readily transmit disease through contaminated hands, toys, floors and tables.

The result is the spread of disease not only among these children but also to other children and adults in their families.

Three "day care diseases" are of greatest concern:

* Hepatitis A, once called "infectious hepatitis." Day care has become the main source of this debilitating liver disease in some cities.

* Serious, sometimes dehydrating diarrheal illnesses like shigellosis, giardiasis and rotaviral disease. A 9-month-old boy died of one of these diseases at a migrant labor camp last July. A month-long epidemic attacked 90 children and adults at the camp's day care center.

* The grave, sometimes fatal bacterial disease, hemophilus influenzae or "H flu" (a misnomer; it is not related to influenza, though it was once thought to be). It typically causes meningitis--brain inflammation -- or pneumonia or both, or other problems. H flu's annual incidence among all children under 5 has been only 2.7 to 4 per 10,000. But it is fatal in 2 to 7 percent of cases, it can cause permanent neurologic damage in 20 to 50 percent more, and the incidence may be increasing now with exposure of more children.

Outbreaks of scarlet fever, tuberculosis, respiratory syncytial disease, measles and other ailments have also been linked to day care exposure.

The problem is most common in the South and southwestern states, where many large day care centers have sprung up in response to population growth and working mothers. Infections have also been commonest in large centers rather than small non-profit centers or small "family homes"--private homes where a mother of a few children cares for a few more.

Children 3 and under, even moreso 2 and under, are more at risk than older, toilet-trained children. Toilet training usually occurs at about 2 1/2.

The infant's co-offender is the careless, untrained or overworked day care worker. Dr. Larry Pickering of the University of Texas in Houston, an authority on diarrheal diseases, tells of crowded, understaffed centers, centers without washbasins near their diapering areas, workers who fail to wash their hands after diaper changes and even lack of soap and towels. Some centers fail to send sick children home.

In one San Antonio center, nearly half the workers had no training or orientation in health or child care. "Unfortunately," Pickering said, day care workers are often seen "as little more than babysitters . . . . This is reflected in their salaries which in the '70s were below the poverty level for two-thirds of staff."

Such conditions are so common, he said, that we risk "providing mere warehouses for our children."

There are also good centers, large and small, profit-making and non-profit. But no day care setting is immune to disease. And the fact that there have been fewer reports from northern states may to a large extent mean authorities "just haven't looked as thoroughly," Pickering believes.

"We almost certainly" have hardly begun to recognize the extent "because no one has made the studies," said Dr. William Rodriguez of Childrens Hospital here. "Day care transmission is probably one of the whole nation's major public health problems. Recognizing it is essentially a matter of awareness. Many centers look the other way."

Dr. Michael Osterholm, infectious disease chief in the Minnesota Health Department, played host this month to several interested experts. They plan a national conference in early 1984, to be preceded by studies to try to attach numbers to their impressions and "find out just what's happening."

"In terms of cost alone, what's happening has to be tremendous," he said. "It costs in medical care. It costs work time of sick parents. Children's illnesses are the major cause of lost time for working women. We don't even know how to measure all the costs."

Doctors and parents have always known that when children get together, they pass on to each other their colds and respiratory illnesses. That is still happening. But it is no longer a matter of a few colds.

Day care hepatitis often goes unrecognized because when young children get it, they are often just mildly ill. They may show no symptoms. They usually don't get jaundiced, like adults.

"But when parents get it from them," said Dr. Stephen Hadler of the CDC laboratories in Phoenix, "they're often very sick and weak and away from work for weeks. They often don't know how they got it, and the parents are usually seen by different doctors so no one catches on to the source."

CDC doctors found day care outbreaks in 19 states from 1979 to 1981. Hadler studied 279 Greater Phoenix day centers. Over two years, 85 had outbreaks affecting 3 to 38 children and families and lasting up to 14 months.

Dr. Diane Dwyer of the Oklahoma Health Department calls one incident in her state "particularly explosive." Its outbreak began in a center in July and lasted until October.

"It's a 24-hour center with between 40 and 60 kids as regulars, but it also takes drop-ins anytime," Dwyer said. "In one week we counted 300 children. It has only two rooms, so you usually have 25 or 30 diapered kids in one room.

"We found crowding. No sink near the changing table. And stool all over the place." Eight people were hospitalized, two of them children.

"We know of at least 20 outbreaks in the last year, and there probably have been many more," Dwyer sums up.

Diarrhea is common in babies and toddlers in the best of conditions. Properly treated, it is usually limited. But it can quickly spread, and if its potential seriousness is not recognized, children may become dangerously dehydrated.

That happened to the 9-month-old child of a migrant worker last summer. He died.

He may have been ill when his parents arrived at the migrant camp. But when authorities visited the 246 children at the migrant day care center and school, she found 55 children under age 2 crowded into two rooms. Eighty percent of them got diarrhea; 90 adults and children got sick; 11 were hospitalized.

"After our investigation they opened a new wing with only six cribs in a room and one staff worker for six infants," the doctor in charge of the investigation said. "We recommended that . . . . They were very willing to change."

In a 19-month study of 20 Houston centers, Pickering found nine had 15 diarrhea outbreaks involving 195 persons, including 34 family members.

Yet, he said, the childrens' parents, often dependent on day care of their children to keep their jobs, "would often deny that their children were ill, or encourage their continued attendance."

"H flu" was not generally considered contagious until the past few years. Increased day care of young children has now led to outbreaks, said Dr. Dan Granoff of St. Louis Childrens Hospital, H flu authority.

"We know of several around the country," Granoff's associate, Dr. Eyla Boies, reports."We're investigating three cases in one week from three different centers." One child died.

Since 1981, Minnesota's Osterholm said, "We've had 56 cases, 40 percent in day care centers, with one death."

Meningitis is the disease's most common complication. The damage can last for life.

Household contacts, adults as well as children, can be affected. They have a 1 to 2 percent attack rate. "With this disease, that's a lot," said Dr. Claire Broome of CDC.

"Day care centers are important; they allow mothers to work," said Boies. She and other health workers agreed that day care can be safe and already is as safe as possible in many centers.

James Strickland of Austin, Tex., president of the Day Care Council of America, a major day care industry group, is helping plan the 1984 conference on health standards. "We've developed a growth phenomenon," he said, "and any time you begin to have changes in society, it behoves all those involved to get together and plan." He thinks "we're a little bit ahead" of the problem "because we're catching it" early enough so "it's more a matter" of preventing future problems than dealing with disasters that have already occurred.

Pickering nonetheless sees a problem today. To a large extent, he said, it consists of weak or unenforced regulations, and "hasty, sporadic and incomplete" inspections in some places.

Some authorities accordingly favor federal regulation or at least a model federally sponsored code for day care size and staffing. New federal regulations were issued near the end of the Carter administration, and a model code was being developed. Both were canceled by President Reagan in favor of local initiative.

The safest place for toddlers, some authorities think, is the small center or family home with half a dozen children or fewer. Still, informed persons give good marks to a large place like the non-profit Rosemount Day Care Center in the District's Mount Pleasant area.

"We have 135 kids, about half 3 and under, and infants from six weeks up," said Jan Yocum, Rosemount's director. "Yes, we have the usual problems, respiratory infections, ear infections, to some extent diarrhea. But we're able to keep them under control.

"Our largest group for 2-year-olds is eight children. For infants under 2, we have a 1-to-3 staff-child ratio, for 2-and-3-year-olds, one to every four or five depending on the children's development.

"We have very strict sanitary procedures, particularly washing hands before and after changing diapers, separate diapering tables and fresh paper on the table for every change. We also think when you have at least 15 children under 3, you need a registered or practical nurse, which we have."

Parents can help, said Pickering. "When they're choosing a center, they should visit it when they're not expected. They should talk to the director and teachers. They should look for cleanliness and safety."

Parents should also act quickly, calling their doctor and alerting the center, when a child is sick or feverish.

"It's time for the facts to be known." Osterholm said. "A lot of doctors and families and day care and health people still aren't aware of this situation."