Strep throat is caused by *Streptococcus pyogenes*, a bacterium also known as group A strep (GAS). It’s a round, gram-positive bacterium that grows in chains and is responsible for an estimated 20,000 to 27,000 serious infections in the United States each year, along with millions of milder throat infections.
What Makes This Bacterium Distinctive
*Streptococcus pyogenes* gets its “group A” label from the Lancefield classification system, which sorts strep bacteria into groups based on a specific carbohydrate on their cell wall. Group A strep is the type most associated with throat infections in humans, though it can also cause skin infections, scarlet fever, and in rare cases, life-threatening invasive disease.
What makes this particular bacterium so effective at causing infection is a surface protein called the M protein. This protein acts like a disguise. It coats the bacterium in a dense layer of the body’s own blood proteins, including antibodies, essentially wearing them backward so immune cells can’t recognize the bacterium as a threat. Normally, antibodies latch onto bacteria and flag them for destruction by white blood cells. The M protein blocks the specific spot on antibodies that immune cells use to grab hold, so the bacteria slip past your defenses largely undetected.
There are more than 200 known types of M protein, which is part of why you can get strep throat more than once. Immunity to one type doesn’t protect you against the others.
How Strep Throat Spreads
Group A strep travels through respiratory droplets. Coughing, sneezing, or sharing food and drinks can all pass the bacteria from one person to another. Someone with untreated strep throat can infect others for two to three weeks. Once you’ve been on antibiotics for 24 hours and your fever is gone, you’re generally no longer contagious.
The bacterium can also live briefly on surfaces like doorknobs or utensils, though direct person-to-person contact is the primary route. Outbreaks are most common in schools, daycare centers, and anywhere people spend time in close quarters.
How Strep Throat Is Diagnosed
Strep throat can’t be reliably diagnosed by symptoms alone because viral sore throats look similar. Clinicians use a scoring tool that weighs five factors: your age, whether you have swollen lymph nodes in your neck, the presence or absence of a cough, whether you have a fever, and whether there’s white or yellow coating on your tonsils. Each factor adds or subtracts a point, producing a score from negative one to five.
A low score (zero or one) means strep is unlikely and testing isn’t needed. A score of two or three means a rapid strep test or throat culture is worth doing. A score of four or five means strep is probable enough that your provider may start treatment based on the clinical picture. The rapid test gives results in minutes by detecting group A strep proteins on a throat swab. If it comes back negative but suspicion is still high, a throat culture (which takes one to two days) can catch cases the rapid test misses.
Treatment and Antibiotic Resistance
Strep throat is one of the few common infections where antibiotic resistance hasn’t become a major problem for first-line treatment. Group A strep bacteria remain fully susceptible to penicillin and amoxicillin, the standard treatments. A typical course lasts 10 days. For people with a penicillin allergy, alternatives are available, though about one in three invasive group A strep infections now show resistance to erythromycin and clindamycin, two common backup antibiotics.
Finishing the full 10-day course matters even after you feel better, which usually happens within a few days. Stopping early doesn’t just risk a relapse. It increases the chance of post-infection complications.
Complications If Left Untreated
Most strep throat infections resolve without lasting harm, especially with antibiotics. But untreated or inadequately treated infections can trigger the immune system to attack the body’s own tissues, leading to two well-known complications.
Acute rheumatic fever typically appears 10 to 28 days after the original throat infection. It causes joint pain, and in some cases, inflammation of the heart valves that can lead to permanent damage. A related condition called Sydenham chorea, which involves involuntary jerking movements, can show up as late as eight months after the initial infection. Rheumatic fever is rare in the United States today, largely because of widespread antibiotic use, but it remains a significant problem in parts of the world with limited healthcare access.
Post-streptococcal glomerulonephritis, a kidney inflammation, usually develops one to three weeks after a throat infection. It can cause dark or bloody urine, swelling in the face and ankles, and high blood pressure. Most cases in children resolve on their own, though adults sometimes have a slower recovery.
Reactive arthritis is another possibility, typically showing up seven to 10 days after the sore throat. It causes joint swelling and pain, most often in the knees and ankles, and generally clears within weeks to months.
Rising Rates of Serious Infections
While routine strep throat remains very treatable, more severe forms of group A strep disease have been climbing. Preliminary CDC data from 2023 show that serious invasive infections, where the bacteria enter the bloodstream, muscles, or lungs, reached a 20-year high. These invasive infections cause an estimated 1,800 to 2,400 deaths per year in the United States. Invasive disease is distinct from a typical sore throat and tends to affect people with weakened immune systems, open wounds, or chronic illnesses. A standard case of strep throat in an otherwise healthy person, treated with a full course of antibiotics, carries very low risk of progressing to anything dangerous.