The baby was running a 101-degree temperature last Saturday morning. By afternoon she was still feverish and vomiting. By evening she was a code blue emergency at Greater Southeast Community Hospital, where she appeared to stop breathing and broke out in seizures.

Doctors soon determined that 5-month-old Sherita Eaton of Varney Street SE was suffering from the same kind of malady that kills an estimated million babies a year in Third World countries -- she was gravely ill because her infant formula had been mixed improperly.

"They told me I was diluting the formula too much," said her mother, Ava Johnson, 29, "but I wad putting four ounces of milk to four ounces of water," as she said the label on the canned Similac concentrate directed. She said that perhaps the formula could have been diluted in Sherita's stomach by the water she was regularly given after feeding.

"What we were seeing and what she said did not correspond," said Pamela Robinson, the hospital's pediatric social worker who counseled Johnson.

"The consensus by doctors and nurses was that the mother was diluting the formula."

Formula-related illnesses arenot common in Washington, but they are frequent enough to concern local pediatricians and health officials, who say mother's milk is free, contains natural immunities and carries less chance of contamination than bottle feeding.

And while the Reagan administration and its critics disagree over the marketing of formula in Third World countries, some critics say that American babies, too, are suffering from formula-related illness and malnutrition.

Wednesday, the day Sherita was released from the hospital, a House subcommittee heard testomony that formula misuse in this country is common among poor, non-English speaking andilliterate women who bottle-feed their babies.

Dr. Allan Cunningham, a ycolumbia University pediatrician, told the subcommittee that as many as 5,000 infant deaths a year could be prevented with universl breast feeding in this country. Formula manufacturers dispute Cunningham's estimate.

The number of illnesses directly related to formula misuse in this country is small compared to places where inpure water, dirty bottles, insects and tropical heat make proper use nearly impossible, according to a year-long study by the public interest law firm, Public Advocates, Inc., which prompted the hearings.

"There is a real danger, among people who can't read the labels, on formula," said Dr. Paul C. Domson, chairman of the pediatrics department at Greater Southeast.

He said formula-related illnesses in infants usually result from formula overdiluted with water or from giving infants spoiled or underdiluted mixtures, which can cause severe dehydration or diarrhea, also resulting in loss of important body chemicals as in Sherita's case.

"She had been drinking diluted formula for a long time and ther wasn't enough sodium in the blood," said Dr. Lloyd Charles, Sherita's doctor. The result was hyponatremia, a dangerously low level of serum sodium, one of the essential chemicals in the blood stream, which made it hard for her to breathe.

Domson said Sherita is one of about three babies a year who are admitted to his hospital with severe illnesses related to formula misuse.

Public Advocates estimates that if all women nursed, nearly $380 million could be save in health care costs for illnesses and formula-related health problems among bottle-fed infants.

Public Advocates said the women most likely to err in mixing formula are the poor and poorly educated who bottle feed in higher proportions than more well-to-do women.

"In Washington, when the families don't mix the formula right it's because they're stressed, they read poorly, they're too young to have the children in the first place, or they haven't told the babysitter [the proper methods] and they gotta go to work," said Dr. Ann Barnet, a Children's Hospital pediatrician and president of The Family Place, a parent education center in Adams-Morgan.

"We commonly see infants that are not being fed the formula properly, but they don't get into trouble unless the child gets sick," said Dr. Ortesia Barr, a pediatrician at Shaw Community Health Center. "They have a lot of tolerance."

Johnson said Sherita, her third child, was a 6-pound, 4-ounce baby who now weighs 12 1/4 pounds and who had "never been sick" until last week. She said her baby "shivered like she was cold. Her eyes rolled back up into her head and she seemed to stop breathing. It scared me.

"I guess it I had gone to Zayre's she would have died," Johnson said.

Charles. Sherita's pediatrician, said the infant probably was not close to death, although she was getting insufficient oxygen, and "all the functions were interrupted."

Barr and several other District doctors said formula-related illnesses as severe as Sherita's are rare, because most bottle-feeding mothers use the ready-to-use canned form.

The Public Advocates study urged increased efforts to encourage breast feeding among lower-income women. More than 90 percent of middle- and upper-income women who give birth in private hospitals already choose to nurse their babies, the study said.

The rate of breast feeding among black women, who are more likely to deliver in large public hospitals, has dropped from 80 percent to 20 percent over the latst 30 years. Often hospital policy of separating mother and baby soon after birth and inadequate support for breast feeding cause women to use formula, the study concluded.

At D.C. General Hospital, where about 1.400 of the city's 20,000 annual births occur, only one mother in 20 breast feeds, the report said. Dr. Stanley Sinkford, the hospital's chief of pediatrics, said he and his colleagues "certainly try to encourage breast-feeding, but in the lower socio-economic groups it's not very well accepted. I don't know what else we can do."

Dr. Bette Catoe, a District pediatrician for 25 years, said among her clients, who range from those with "no income to millionaires," there is incresing guilt among parents who bottle-feed as a result of the "media blitz," urging breast feeding.

"Particularly in a black practice, most of our mothers have to work, either for a second income or as single mothers," she said. "It is difficult for the non-professional mother to find that flexibility."