What Causes Group B Strep?

Group B Streptococcus (GBS) is a common bacterium that lives in the bodies of about one in four healthy adults. It typically resides in the digestive tract, urinary tract, and the vaginal tract in women. The cause is simply the presence of this specific bacteria, which most people carry without ever getting sick. It is not an infection you “catch” from someone else like a cold — it is a normal part of your body’s bacterial landscape.

How Do People Get Group B Strep?

You do not get Group B Strep from the environment or from casual contact. Research shows that GBS is part of the normal human microbiome. It lives in the gut and can move to the urinary or genital areas. For most adults, this causes zero problems.

The bigger question for pregnant women is timing. If a woman carries GBS in her vagina or rectum during labor, the baby can be exposed during birth. This is not a new infection — she likely carried it before pregnancy. The CDC reports that about 10 to 30 percent of pregnant women test positive for GBS at some point. Most never know it because there are no symptoms.

What Causes Group B Strep in Newborns?

Newborns get GBS when they pass through the birth canal of a mother who carries the bacteria. The baby can swallow or inhale the fluid containing GBS. This exposure happens during labor and delivery, not during pregnancy itself.

The risk is real but relatively low. Studies have found that without preventive treatment, about 1 to 2 percent of babies born to GBS-positive mothers will develop an infection. With proper screening and antibiotics during labor, that number drops to less than 0.1 percent. The American College of Obstetricians and Gynecologists recommends universal screening for all pregnant women between 36 and 38 weeks of pregnancy.

Early-onset GBS disease appears within the first week of life. Late-onset disease can appear between one week and three months. The causes are the same — exposure to the bacteria — but late-onset cases sometimes come from sources other than the mother.

What Are the Risk Factors for GBS in Adults?

Most adults who carry GBS never develop an infection. But certain conditions increase the risk of GBS causing illness. These include:

• Diabetes, especially if blood sugar is not well controlled
• Being over 65 years old
• Having a weakened immune system from illness or medication
• Cerebrovascular disease or heart disease
• Having a urinary catheter in place
• Bedsores or chronic skin ulcers

In non-pregnant adults, GBS can cause urinary tract infections, bloodstream infections, pneumonia, and skin infections. The same bacteria that lives harmlessly in the gut can become dangerous when it gets into the wrong part of the body. Research published in the journal Clinical Infectious Diseases found that GBS causes about 25,000 invasive infections in non-pregnant adults each year in the United States.

How Is Group B Strep Diagnosed and Treated?

Diagnosis happens through a simple swab test. During the third trimester of pregnancy, a healthcare provider swabs the vagina and rectum. The sample is sent to a lab to grow the bacteria. Results take a few days.

There is no vaccine for GBS as of 2026. Treatment for pregnant women who test positive is intravenous antibiotics during labor. Penicillin is the standard choice. For women allergic to penicillin, alternatives like cefazolin or clindamycin are used depending on sensitivity testing.

Treatment is not given before labor because the bacteria can return quickly. The goal is to have antibiotics in the mother’s bloodstream at the time of delivery so the baby gets protection during birth. The CDC states that this approach prevents 80 percent of early-onset GBS infections in newborns.

For non-pregnant adults with an active GBS infection, oral or intravenous antibiotics are prescribed. The specific antibiotic depends on the type and location of the infection.

What Causes Group B Strep to Become Dangerous?

GBS becomes dangerous when it moves from its normal home in the gut or vagina into sterile parts of the body like the bloodstream, the lining of the brain, or the lungs. This is called invasive GBS disease.

In newborns, the immature immune system cannot always keep the bacteria contained. The bacteria can multiply rapidly in the blood or spinal fluid. This is why early-onset GBS can cause sepsis, pneumonia, or meningitis.

In adults, invasive GBS typically happens when there is a break in the skin or a medical condition that weakens immune defenses. A diabetic foot ulcer, for example, can become infected with GBS and then spread to the bloodstream. The table below compares how GBS behaves in different populations.

Common Misconceptions About Group B Strep

One widespread myth is that GBS is a sexually transmitted infection. It is not. GBS is part of the normal bacteria in the gut and can be found in the genital area without any sexual activity. Testing positive does not mean a person has an STI.

Another misconception is that having GBS means you are “dirty” or have poor hygiene. This is false. Carrying GBS has nothing to do with cleanliness. It is simply a matter of which bacteria happen to live in your body. About 25 percent of healthy women carry it at any given time.

Some people believe that if you test negative early in pregnancy you are safe. Testing is only accurate close to delivery because GBS can come and go. This is why the standard screening window is 36 to 38 weeks. A negative result at 20 weeks does not guarantee a negative result at delivery.

There is also confusion about whether GBS can be treated during pregnancy with oral antibiotics. This does not work well. Oral antibiotics can temporarily reduce GBS levels, but the bacteria typically return within weeks. The only proven effective approach is intravenous antibiotics during active labor.

What to Avoid When Managing GBS Risk

Avoid relying on home remedies or unproven supplements to “clear” GBS. Some people report success with garlic, probiotics, or vitamin C, but strong clinical evidence is limited. The American College of Obstetricians and Gynecologists does not recommend any alternative treatments for GBS in pregnancy.

Do not skip the screening test. It is a simple swab that takes seconds and provides critical information. About 40 percent of women who have a baby with early-onset GBS disease had no prenatal screening. Missing the test means missing the chance for preventive antibiotics.

Avoid assuming you are immune because you tested negative in a previous pregnancy. GBS status can change between pregnancies. Each pregnancy requires its own screening at 36 to 38 weeks. The bacteria can appear or disappear without any clear reason.

Do not panic if you test positive. The vast majority of GBS-positive women who receive antibiotics during labor deliver healthy babies. The risk of a serious infection in the newborn is very small with proper medical care.