To his parents, Jeffrey Grant seemed to be a normal, healthy, week-old baby when, one morning last September, he started acting fussy and developed a 100-degree fever. As his worried parents drove him to the pediatrician's office -- five minutes from their Arlington home -- he suddenly turned green.

The doctor, who saw he was in shock, picked him up and dashed next door to Arlington Hospital's emergency room. Despite massive doses of antibiotics, Jeffrey died several hours later of a bacterial infection at Georgetown University's neonatal intensive care unit.

"It was incredibly fast," recalled his father Robert Grant. "In the space of 20 minutes, we went from being worried about the fact that he had a fever to being told that he was probably going to die."

Although little known to the general public, group B streptococcus -- the illness that killed Jeffrey Grant -- is the most common cause of infection in infants less than two months old, according to the Centers for Disease Control. The reason, doctors say, is that newborns lack immunity to many types of infections.

An estimated one in five pregnant women unknowingly carries the bacteria in her genital tract, where it is transmitted to infants either before or during delivery. "The vast majority of those women and their babies will remain healthy," said Carol Baker, who studies streptoccocus B at Baylor College of Medicine in Houston. But others, like the Grants, are not so lucky, for reasons that puzzle doctors.

An estimated 11,000 babies this year will contract a full-blown strep B infection, which can quickly lead to sepsis -- the systemic infection of the blood -- or meningitis, a brain infection. About 20 percent or 2,200 infants will die after contracting strep B; those who survive a bout of meningitis have about a 25 percent chance of sustaining some permanent neurological damage, according to Kenneth M. Boyer, a pediatrician who does strep B research at Rush-Presbyterian St. Luke's Medical Center in Chicago.

A different strain of the same bacterial illness that killed Muppets creator Jim Henson, group B strep is 20 times more likely than better-known congenital illnesses such as herpes, syphilis and rubella to afflict newborns. Yet screening tests for strep B have lagged far behind those used to detect other diseases, largely because the infection causes no symptoms in adults, where it can thrive for years without detection and usually without causing any harm. Babies who contract the illness typically develop a fever and respiratory problems; the illness can progress quickly, as it did in Jeffrey Grant's case.

"The problem is when do you screen and whom do you screen?" said Richard H. Schwarz, president-elect of the American College of Obstetricians and Gynecologists.

So many pregnant women are carriers of strep B that detecting it means testing virtually all expectant mothers, a measure many doctors favor. But only those who meet certain criteria -- who develop a fever during labor, go into labor prematurely or whose amniotic fluid breaks before labor begins -- likely would be treated with intravenous antibiotics as a preventive measure.

Of the estimated 4 million women who give birth annually, about 500,000 would receive such antibiotic therapy, according to Richard Sweet, president of the Infectious Disease Society for Obstetrics and Gynecology and vice-chairman of obstetrics and gynecology at the University of California at San Francisco.

Although it is difficult to know which babies are more likely to contract strep B, studies have shown that those who are premature are at greater risk, as are those born to mothers who develop a fever during delivery or who are born more than 12 hours after the amniotic fluid ruptures.

The disease affects mothers as well. Group B strep infections account for 45,000 cases annually of postpartum endometritis -- an inflammation of the uterine lining which is treated with antibiotics and is the second most common cause of bacteremia, a blood infection following a Caesarean section. The estimated annual cost of treating these problems totals about $269 million according to a National Institutes of Health panel.

Boyer estimated that testing costs about $30 per person and that antibiotic treatment costs an additional $50.

Increasingly physicians, concerned about the toll and cost of strep B, are advocating a more aggressive approach to preventing and treating it.

Later this year, the Infectious Disease Society for Obstetrics and Gynecology will meet to try to develop guidelines for doctors. "We need to come up with recommendations of what to do," said Sweet.

One approach may be to screen pregnant women to determine if they harbor the bacteria; those who do could be treated with intravenous antibiotics during labor and delivery if they run a fever, if their water breaks before delivery or if they go into premature labor. A study of 160 pregnant women by researchers at Chicago's Rush-Presbyterian found that screening prevented infection in newborns.

Other doctors advocate routinely testing women at about 37 weeks of pregnancy, just a few weeks before the expected 40-week due date.

But that may miss women who give birth prematurely.

"Somewhere between 70 to 90 percent of the mortality {from strep B infections} is in premies," said Sweet, who favors screening women at about 26 to 28 weeks of pregnancy and then administering antibiotics to carriers during high-risk deliveries.

Widespread screening, however, will miss women who don't seek prenatal care. Current tests to detect strep B can be performed in about 18 hours -- not fast enough to detect the bacteria before many women in labor give birth.

"What we really need is a rapid diagnostic test that we can use to find out in five minutes whether a woman is a group B strep carrier," Sweet said.

A vaccine might also help. "You could target a vaccine to all women of childbearing age," said Baylor's Baker, who said that it could theoretically be coupled with a vaccine for tetanus, which is sometimes given to women during pregnancy.

Until then, early detection and rapid treatment can help prevent brain damage and infant deaths. Several weeks before Jeffrey Grant died, newborn Kyle Harsanyi started having breathing problems in the Arlington Hospital nursery. A nurse alerted his doctor, who quickly took a culture from the baby and started intravenous antibiotics. Harsanyi was transferred to the neonatal intensive care unit, where he remained for three days, then spent another week in the regular nursery receiving antibiotics intravenously. He recovered completely.

"They caught it so early," said his relieved mother, Sheri Harsanyi, a nurse at Arlington Hospital whose son will celebrate his first birthday next month. "We were really lucky."Screening was the only thing that doctors say could have helped prevent Jeffrey Grant's death. His older brother, now 4, showed no signs of the disease. Jeffrey's delivery was normal in all respects, and he was not premature.

It wasn't until after he died that doctors tested Jeffrey's mother and found that she carries group B streptococcus.

For more information, contact the Group B Streptococcus Association, P.O. Box 16515, Chapel Hill, N.C. 27516.