Group A strep is generally the more dangerous of the two types, causing a wider range of severe and potentially fatal infections in otherwise healthy people. But “worse” depends on who you are: Group B strep is the bigger threat for newborns and can be deadly in that narrow window around birth. For the general population, though, Group A strep is responsible for more invasive infections, higher mortality rates in its most serious forms, and a growing number of cases each year.
What the Two Types Actually Are
When people say “strep,” they usually mean Group A Streptococcus (also called Streptococcus pyogenes). This is the bacterium behind strep throat, scarlet fever, and skin infections like impetigo. It spreads easily between people through respiratory droplets and direct contact, and it can infect anyone at any age.
Group B Streptococcus (Streptococcus agalactiae) is a different species entirely. It lives naturally in the digestive and genital tracts of many healthy adults without causing any symptoms. It becomes a problem primarily during childbirth, when a mother can pass it to her baby, and in older adults with weakened immune systems.
How Group A Strep Can Turn Deadly
Most Group A strep infections are mild. Strep throat is uncomfortable but clears up with antibiotics in a few days. The real danger comes when the bacteria invade parts of the body they don’t normally reach, like the bloodstream, muscles, or lungs. These invasive infections are uncommon, but they can escalate fast.
The two most feared complications are necrotizing fasciitis, sometimes called “flesh-eating disease,” and streptococcal toxic shock syndrome (STSS). Necrotizing fasciitis destroys muscle and fat tissue and kills roughly 20% of patients even with aggressive treatment. STSS causes a rapid collapse in blood pressure and organ damage to the kidneys, liver, and lungs. Its fatality rate is around 60%.
These numbers make Group A strep one of the more lethal bacterial infections when it turns invasive. In a CDC analysis of meningitis cases from 1997 to 2022, Group A strep meningitis had a case fatality rate of 19.4%, higher than meningitis caused by Group B strep, pneumococcal bacteria, or any of the other major bacterial causes.
The Threat Group B Strep Poses to Newborns
Group B strep is a serious concern in a very specific context: the hours surrounding birth. If a mother carries the bacteria in her birth canal, the baby can be exposed during delivery. In newborns, GBS can cause bloodstream infections, pneumonia, and meningitis, all of which can be life-threatening for an infant whose immune system is still developing.
This is why pregnant women are routinely screened for Group B strep late in pregnancy, typically between weeks 36 and 37. If the test is positive, antibiotics given during labor dramatically reduce the chance of the baby becoming infected. This screening protocol has made early-onset GBS disease in newborns far less common than it once was, but it hasn’t eliminated it entirely.
Long-Term Damage From Group A Strep
One thing that sets Group A strep apart is what can happen after the infection itself is gone. The body’s immune response to the bacteria can trigger complications that affect the heart and kidneys weeks later. Rheumatic fever, which can permanently damage heart valves, is the most well-known of these. It’s now rare in wealthy countries thanks to antibiotic treatment of strep throat, but it remains a major cause of heart disease in lower-income regions.
Post-streptococcal glomerulonephritis is another delayed complication. It’s an inflammatory condition affecting the kidneys that can develop about 10 days after strep throat or scarlet fever, or about 3 weeks after a strep skin infection. Most people recover within a few weeks, but in rare cases, particularly in adults, it can lead to lasting kidney damage or kidney failure. Group B strep does not carry these same risks of post-infection autoimmune complications.
Rising Case Numbers
Invasive Group A strep infections have been climbing in the United States since 2014. Preliminary 2023 data from the CDC indicate that serious Group A strep infections reached a 20-year high, with estimates of 20,000 to 27,000 invasive cases occurring each year over the past five years. The reasons for this increase aren’t fully understood, but the trend means more people are encountering the severe end of this bacterium’s range.
Antibiotic Treatment Differences
Both types of strep are still treatable with penicillin, which is good news. Group A strep has remained remarkably susceptible to penicillin despite decades of use, making it one of the more straightforward bacterial infections to treat when caught early.
Group B strep, on the other hand, is showing worrying resistance to backup antibiotics. More than 43% of invasive GBS infections are resistant to clindamycin, and over 58% are resistant to erythromycin. This matters most for patients with severe penicillin allergies, who need those alternative drugs. When those options stop working, treatment becomes more complicated.
Which One Should You Worry About More
For the average person, Group A strep is the greater overall threat. It’s more common, more easily spread, more likely to cause severe disease in healthy people, and capable of triggering lasting heart and kidney damage even after the initial infection resolves. Its most extreme complications carry fatality rates that few other common bacteria can match.
Group B strep is a more targeted danger. If you’re pregnant, it’s a critical concern because of the risk to your baby, but screening and prevention protocols have made outcomes much better than they used to be. For older adults or those with chronic health conditions like diabetes, GBS can also cause serious invasive infections, though these are less common than invasive Group A strep cases. In terms of sheer breadth of harm across all age groups, Group A strep is the worse of the two.