A type of bacteria that is commonly present in the intestine or a person's genital tract causes Streptococcus group B, according to Mayo Clinic. This type of bacteria is present in many healthy people, although doctors don't know why some people develop a serious infection while others do not.
Babies can contract group B strep during vaginal delivery if the bacteria is present in the mother, and some groups of people, such as older adults, can develop more serious group B strep infections, states Mayo Clinic. Group B strep is typically not harmful for adults, although people with liver disease, diabetes or other chronic medical issues can develop serious infections from this bacteria. A woman who is pregnant can protect her unborn child during delivery by getting a screening during the third trimester of pregnancy.
In the United States, group B strep is the most noted cause of meningitis in newborns, according to the CDC. While antibiotics during delivery do not prevent 100 percent of all transmissions of the bacteria, a woman who has group B strep and undergoes antibiotic treatment only has a one in 4,000 chance of passing the bacteria to her child. A pregnant woman who has group B strep and does not receive antibiotics has a one in 200 chance of passing the bacteria to her baby during delivery.