Newborns can contract GBS during pregnancy, or from the mother's genital tract during labor and delivery. GBS is the most common cause of life-threatening infections in newborns, including pneumonia and meningitis. About one out of every 100 to 200 babies whose mothers carry GBS develop symptoms of GBS disease. Nearly 75 percent of the cases of GBS disease among newborns occur in the first week of life, called early-onset disease. Premature babies are more susceptible to GBS infection than full-term babies.

Although it is very rare, GBS infection may also develop in babies one week to several months after birth, called late-onset disease. Meningitis is more common with late-onset GBS disease. Only about half of late-onset GBS disease among newborns comes from a mother who is a GBS carrier. The source of infection for others with late-onset GBS disease is unknown.

In infants, GBS may be cultured from samples of sterile body fluids, such as blood or spinal fluid. Most newborns with GBS infection have symptoms in the first few hours after birth. Symptoms are related to the body systems that are affected by the infection and may include breathing problems, changes in blood pressure, or neurologic problems such as seizures.