Group B strep (GBS) is a type of bacteria that naturally lives in the digestive and genital tracts of roughly 1 in 4 adults. Most people who carry it have no symptoms and never know it’s there. You don’t “catch” it the way you catch a cold. Instead, GBS is part of the normal bacterial population of the human body, and it only becomes a medical concern under specific circumstances: during childbirth, when it can pass to a newborn, or when an adult’s immune system is weakened enough that the bacteria cause active infection.
Where GBS Lives in the Body
GBS bacteria colonize the gastrointestinal and genital tracts. The intestines are the primary reservoir, and from there the bacteria can spread to the vagina and rectum. This colonization is not an infection. It doesn’t cause symptoms, and it can come and go over time. A person may test positive for GBS one month and negative the next, then positive again later. The bacteria aren’t doing any harm in these locations; they simply coexist with the many other microorganisms that make up your body’s normal flora.
Exactly how people initially acquire GBS or pass it between one another remains unclear. The CDC notes that the way GBS spreads from person to person is “generally unknown,” and there is no evidence that it spreads through food, water, or contact with contaminated objects. It is not considered a sexually transmitted infection, though it can be present in the genital tract. The practical takeaway is that colonization is extremely common, not a sign of poor hygiene, and not something you can reliably prevent.
How Newborns Get GBS
The situation where GBS matters most is pregnancy and childbirth. When a pregnant person carries GBS in their vaginal or rectal area, the bacteria can pass to the baby during labor and delivery as the baby moves through the birth canal. This is called vertical transmission, and it’s the primary cause of early-onset GBS disease in newborns, which appears within the first week of life (usually within hours of birth).
Not every baby exposed to GBS during delivery gets sick. The vast majority don’t. But for those who do, the consequences can be serious: pneumonia, bloodstream infections, and meningitis. That’s why screening and prevention during pregnancy are so important.
There’s also a second form called late-onset GBS disease, which develops between one week and three months of age. The transmission source for late-onset disease is less well understood. It may come from the mother, from other caregivers who carry the bacteria, or from the infant’s own colonization acquired at birth. Because the pathway is unclear, late-onset disease is harder to prevent with the strategies currently used for early-onset cases.
Screening During Pregnancy
Because GBS colonization causes no symptoms, the only way to know if you carry it during pregnancy is through a screening test. The American College of Obstetricians and Gynecologists recommends that all pregnant people be tested between 36 weeks and 37 weeks, 6 days of gestation, regardless of how they plan to deliver. The test itself is simple: a swab of the vagina and rectum, sent to a lab for culture.
Timing matters. GBS colonization fluctuates, so a test done too early in pregnancy may not reflect your status at the time of delivery. The late third-trimester window gives the most accurate picture of whether bacteria will be present when labor begins.
If the test comes back positive, you’ll receive IV antibiotics during labor. This doesn’t eliminate GBS from your body permanently, but it dramatically reduces the amount of bacteria present in the birth canal at the time of delivery, lowering the risk that your baby will be exposed. Two groups of pregnant people skip the screening step and go straight to receiving antibiotics during labor: those who had GBS bacteria in their urine during the current pregnancy, and those who previously had a baby affected by GBS disease.
How Adults Get GBS Infections
Outside of pregnancy, GBS can cause active infections in adults, particularly those with weakened immune defenses. The bacteria that have been quietly colonizing the body can enter the bloodstream or other tissues when the immune system can no longer keep them in check. This leads to infections like bloodstream infections (bacteremia), skin and soft tissue infections, pneumonia, bone and joint infections, and meningitis.
Several medical conditions raise the risk significantly:
- Diabetes, which impairs the body’s ability to fight infection
- Cancer or a history of cancer, especially when treatment suppresses immune function
- Heart disease and congestive heart failure
- Obesity
Age is also a major factor. Adults 65 and older face a higher risk of GBS disease compared to younger adults. The combination of aging immune defenses and chronic health conditions makes this group particularly vulnerable. In fact, GBS disease in non-pregnant adults has been increasing in recent decades, largely driven by the growing population of older adults living with conditions like diabetes.
It’s worth emphasizing that these adult infections are not “caught” from someone else. They arise from bacteria already present in the person’s own body. The shift from harmless colonization to active disease is driven by changes in the host’s immune status, not by a new exposure.
Why You Can’t Prevent Colonization
Because GBS is part of the normal human microbiome and its transmission routes aren’t fully understood, there’s no reliable way to prevent yourself from becoming colonized. Antibiotics can temporarily clear the bacteria, but they return. This is why the prevention strategy for newborns focuses on the narrow window of labor and delivery rather than trying to eliminate GBS during pregnancy.
For non-pregnant adults, there’s no routine screening for GBS and no preventive treatment. Managing the chronic conditions that increase risk, like keeping blood sugar controlled if you have diabetes, is the most practical step you can take to reduce your chances of GBS turning from a harmless passenger into an active infection.