Group B streptococcus (GBS) is a bacterial infection that can cause serious illness in newborns, leading to conditions such as sepsis and pneumonia. Diagnosis involves growing the bacteria from a baby's body fluids, and treatment typically requires antibiotics delivered intravenously. Preventive measures include screening pregnant women at 35-37 weeks gestation and administering antibiotics during labor for those who test positive for GBS to reduce transmission to newborns.