Strep throat is a bacterial infection, not a viral one. It’s caused by a specific species of bacteria called group A Streptococcus, and that distinction matters because it changes how the infection is treated, how long you’re contagious, and what can happen if it’s left alone.
What Causes Strep Throat
The bacterium responsible is Streptococcus pyogenes, commonly referred to as group A strep. These are round-shaped bacteria that grow in chains and are classified based on how they break down red blood cells in lab testing. When a doctor sends a throat swab to the lab, this is the organism they’re looking for.
This is an important distinction because most sore throats are actually viral. Viruses cause the majority of pharyngitis cases in both children and adults. Strep bacteria account for a meaningful share of sore throats in school-age kids (roughly 20 to 30 percent), but a smaller proportion in adults. So while strep throat itself is always bacterial, the sore throat you’re experiencing at any given moment is statistically more likely to be viral.
How to Tell Strep Apart From a Viral Sore Throat
Strep throat and viral sore throats can feel similar at first, but several clues point in different directions. Symptoms that suggest a virus rather than strep include a cough, runny nose, hoarseness, and pink eye. If you have a sore throat alongside those cold-like symptoms, strep is less likely.
Doctors use a clinical scoring system to estimate the probability that a sore throat is strep before running any tests. The score goes up if you have swollen or pus-covered tonsils, tender swollen lymph nodes in the front of your neck, a fever above 100.4°F, and no cough. Being between ages 3 and 14 also adds a point, since strep is most common in that age group. The more of these features you have, the more likely it is bacterial.
That said, no combination of symptoms can confirm strep on its own. Testing is needed.
How Strep Throat Is Diagnosed
The two main tests are a rapid antigen test (the quick in-office swab) and a throat culture grown in a lab. The rapid test gives results in minutes but catches about 86% of true strep cases, meaning it misses roughly 1 in 7 infections. Its specificity is around 95%, so a positive result is highly reliable. Because the rapid test can produce false negatives, a follow-up throat culture is sometimes sent when the quick test comes back negative, especially in children. Throat cultures are more accurate but take one to two days for results.
Why the Bacteria vs. Virus Question Matters for Treatment
Antibiotics work against bacteria but do nothing for viruses. That’s the core reason this distinction matters. A viral sore throat resolves on its own with rest and symptom management. Strep throat also often improves without treatment, but antibiotics are prescribed for two important reasons: they shorten the illness, and they prevent rare but serious complications.
The standard treatment is a 10-day course of penicillin or amoxicillin. Group A strep bacteria have not developed resistance to either of these antibiotics, which makes them reliably effective. If you’re allergic to penicillin, several alternatives exist. Your doctor will typically choose from a different class of antibiotics, though it’s worth noting that about 1 in 3 invasive group A strep infections now show resistance to some of those backup options.
Once you start antibiotics, you’re generally no longer contagious within 12 hours. That’s the standard guideline for when children can return to school or daycare. Without antibiotics, you can spread the bacteria for days or even weeks while symptomatic.
What Happens if Strep Goes Untreated
Most people recover from strep throat even without antibiotics, but leaving it untreated carries real risks. The most concerning complication is rheumatic fever, which can develop one to five weeks after the infection. Rheumatic fever is not the bacteria spreading further. It’s an immune system overreaction: your body’s defenses, primed to fight the strep bacteria, mistakenly attack healthy tissue in the heart, joints, brain, or skin.
If rheumatic fever isn’t caught and treated quickly, it can progress to rheumatic heart disease, which damages the valves between the heart’s chambers. Severe cases can require heart surgery and can be fatal. This is the primary reason doctors take strep throat seriously even though the sore throat itself is usually manageable.
Kidney inflammation is another possible complication, where the immune response to the strep infection damages the small filtering units in the kidneys. This condition typically resolves on its own but can occasionally cause lasting problems.
Who Gets Strep Throat Most Often
Strep throat peaks in children between ages 5 and 15. It spreads through respiratory droplets (coughing, sneezing, sharing drinks) and is common in schools, daycare centers, and households with young children. Adults can absolutely get strep, but it accounts for a smaller percentage of their sore throats compared to children. In adults over 45, strep becomes even less common.
Children under 3 rarely get classic strep throat. They can carry the bacteria, but the typical strep presentation with sudden sore throat, fever, and swollen tonsils is uncommon in toddlers. When very young children do get group A strep infections, the symptoms often look different, more like a prolonged runny nose and irritability than a distinct sore throat.