Strep A, short for Group A Streptococcus, is a type of bacteria responsible for some of the most common infections in humans, from strep throat to skin infections like impetigo. The bacterium’s scientific name is Streptococcus pyogenes, and while most infections it causes are mild and easily treated with antibiotics, it can occasionally lead to serious, even life-threatening illness.
How Strep A Spreads
Strep A bacteria live in the throat and on the skin. They spread primarily through close contact with an infected person, typically via respiratory droplets produced by coughing or sneezing. Sharing cups, utensils, or direct skin-to-skin contact can also transmit the bacteria. Household spread is especially common: if one person in your home has strep throat, others are at elevated risk.
After exposure, it usually takes two to five days before symptoms appear. During that window and while symptoms are active, an infected person is contagious. Once appropriate antibiotics are started, most people are no longer contagious after about 24 hours, though they still need to complete the full course of treatment.
Common Strep A Infections
The vast majority of Strep A infections fall into a few familiar categories:
- Strep throat: The most well-known Strep A infection. It causes a sudden, severe sore throat, pain when swallowing, fever, red and swollen tonsils (sometimes with white patches), and swollen lymph nodes in the front of the neck. Unlike a typical cold, strep throat usually does not come with a cough, runny nose, or congestion.
- Scarlet fever: Essentially strep throat accompanied by a distinctive red, sandpaper-textured rash that typically starts on the chest and spreads outward. It’s most common in children and is treated with the same antibiotics as strep throat.
- Impetigo: A skin infection that produces red sores, usually around the nose and mouth, which rupture and form a honey-colored crust. It’s highly contagious and common in young children.
These infections are uncomfortable but rarely dangerous when treated promptly.
How Strep Throat Is Diagnosed
Diagnosing strep throat based on symptoms alone is unreliable because viral sore throats look similar. Clinicians often use a scoring system that considers four factors: fever of 38°C (100.4°F) or higher, absence of cough, swollen lymph nodes at the front of the neck, and swelling or white patches on the tonsils. Each factor adds one point on a scale of zero to four. A higher score means a bacterial cause is more likely, but testing is still needed to confirm.
The most common test is a rapid antigen detection test, often called a “rapid strep test.” A swab of the back of your throat gives results in minutes. These tests are very good at ruling out strep when negative in adults, with a specificity around 95%, meaning false positives are rare. However, sensitivity sits around 86%, so roughly 14% of true infections get missed. For children, many guidelines recommend following up a negative rapid test with a throat culture, which takes one to two days but is more accurate.
Treatment and Recovery
Strep A infections respond well to common antibiotics. Penicillin and amoxicillin are the first choices for strep throat. A typical course lasts 10 days, even though you’ll likely feel better within two or three days. Finishing the full course matters because stopping early increases the risk of the infection returning and of complications developing.
If you’re allergic to penicillin, several alternatives are available, including certain types of antibiotics in the cephalosporin and macrolide families. Your provider will choose the best option based on the type of allergy you have. One trend worth noting: antibiotic resistance in Strep A is climbing. Data from surveillance across 10 U.S. states shows that the percentage of Strep A isolates resistant to macrolides and clindamycin rose from about 13% in 2013 to 33% in 2022. Penicillin and amoxicillin remain effective, which is one reason they’re still preferred.
Complications to Be Aware Of
Most Strep A infections resolve without problems, but untreated or poorly treated infections can trigger secondary complications weeks later. These aren’t caused by the bacteria still being present. Instead, they result from the immune system’s response to the original infection.
Acute rheumatic fever is the most well-known complication. It can develop after strep throat (not skin infections) and may cause joint pain, heart inflammation, and, in severe cases, lasting damage to heart valves. It’s rare in developed countries today, largely because of widespread antibiotic use, but it remains a concern in parts of the world with limited healthcare access.
Post-streptococcal glomerulonephritis is a kidney condition that can follow either strep throat or a strep skin infection. It typically appears about 10 days after a throat infection or up to three weeks after a skin infection. Symptoms include dark or cola-colored urine, swelling in the face and legs, and reduced urine output. Most cases, particularly in children, resolve on their own, though some require supportive care.
Invasive Strep A: The Serious End of the Spectrum
In rare cases, Strep A bacteria enter parts of the body they don’t normally reach, like the bloodstream, muscles, or lungs. This is called invasive Group A Streptococcal disease (iGAS), and it includes conditions like necrotizing fasciitis (a rapidly spreading soft tissue infection sometimes called “flesh-eating disease”), toxic shock syndrome, and bloodstream infections.
Invasive Strep A is uncommon but has been increasing. According to data published in JAMA, the incidence of invasive GAS infections across surveillance areas in 10 U.S. states more than doubled, rising from 3.6 per 100,000 people in 2013 to 8.2 per 100,000 in 2022. The sharpest increases occurred among adults aged 18 to 64. Certain populations face dramatically higher risk: the estimated rate among people experiencing homelessness surged from 85 to 807 per 100,000 over the same period, and people who inject drugs also carry substantially elevated rates.
Part of this rise is driven by the emergence of bacterial strains that were previously uncommon. Four strain types that collectively made up just 0.3% of invasive isolates in 2013 accounted for nearly 27% by 2022. These newer strains tend to be associated more with skin infections than throat infections, a pattern that continued through and after the COVID-19 pandemic.
Who Is Most at Risk
Strep throat is overwhelmingly a childhood illness, peaking between ages 5 and 15, though adults get it too. Invasive disease follows a different pattern: it’s most common in adults over 65, people with chronic health conditions, those with breaks in the skin (including surgical wounds), and people with weakened immune systems. The recent surveillance data highlights that social and environmental factors, such as homelessness and injection drug use, are among the strongest risk factors for severe disease.
Strep A has no vaccine, so prevention relies on basic hygiene: frequent handwashing, covering coughs and sneezes, and avoiding sharing personal items when someone in your household is infected. Keeping wounds clean and covered also reduces the risk of skin-related Strep A infections.