Group B streptococci are common inhabitants of the human gastrointestinal and genitourinary tracts. Less commonly, they colonize the pharynx. The colonization rate in pregnant women ranges from 15% to 40%. Colonization during pregnancy can be constant or intermittent. Before recommendations for prevention of early-onset group B streptococcal (GBS) disease by maternal intrapartum antimicrobial prophylaxis (see Control Measures) were made, the incidence was 1 to 4 cases per 1000 live births; early-onset disease accounted for approximately 75% of cases in infants and occurred in approximately 1 infant per 100 colonized women. Associated with widespread maternal intrapartum antimicrobial prophylaxis, the incidence of early-onset disease has decreased by approximately 80% to approximately 0.35 cases per 1000 live births in 2005 and now equals that of late-onset disease. Case-fatality rates in term infants range from 3% to 5% but are higher in preterm neonates. Transmission from mother-to-infant occurs shortly before or during delivery. After delivery, person-to-person transmission can occur. Although uncommon, GBS infection can be acquired in the nursery from hospital personnel (probably via hand contamination) or more commonly in the community from healthy colonized people. The risk of early-onset disease is increased in preterm infants born at less than 37 weeks of gestation, in infants born after the amniotic membranes have been ruptured 18 hours or more, and in infants born to women with high genital GBS inoculum, intrapartum fever (temperature 38°C [100.4°F] or greater), chorioamnionitis, GBS bacteriuria during the pregnancy, or a previous infant with invasive GBS disease. A low or an absent concentration of serotype-specific serum antibody also is a predisposing factor. Other risk factors are intrauterine fetal monitoring, maternal age younger than 20 years, and black ethnicity. Rates of early-onset disease increased from 2003 to 2005 among black infants (0.52-0.89 cases per 1000 live births) and decreased among white infants (0.26-0.22 cases per 1000 live births). The period of communicability is unknown but may extend throughout the duration of colonization or of disease. Infants can remain colonized for several months after birth and after Treatment for systemic infection. Recurrent GBS disease affects an estimated 1% to 3% of appropriately treated infants.

The incubation period of early-onset disease is fewer than 7 days. In late-onset and late, late-onset disease, the incubation period from GBS acquisition to disease is unknown. The incidence of GBS disease declines dramatically after 89 days of age, but up to 10% of pediatric cases occur beyond early infancy, and many, but not all, of these are in infants who were born preterm.