Causes and Risk Factors of Strep Throat

Most of the time pharyngitis (sore throat) is caused by viruses. Strep throat is the most common bacterial cause of a sore throat. It is responsible for 15 to 30% of cases in children and 5 to 10% in adults.

While streptococcus infection is the definitive cause of strep throat, there are several factors that can make someone more susceptible to this infection. Knowing about them can help you to decrease your risk of this infection.

strep throat causes and risk factors
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Streptococcal Infection

There are different strains of strep bacteria, the most common of which lead to respiratory and skin infections. The four common types of streptococcal bacteria are A, B, C, and G. Group A Streptococcus (GAS), also known as Streptococcus pyogenes, is the bacteria responsible for strep throat.

Besides strep throat, other common infections caused by S. pyogenes include:

Untreated strep throat can lead to recurrent episodes, or to the more serious but rare complication of rheumatic fever. ​

Even if treated, strep throat can sometimes lead to post-streptococcal glomerulonephritis (PSG), a disease that causes inflammation in the kidneys. Most people completely recover from PSG without long-term complications.

How Streptococcal Bacteria Spreads

S. pyogenes is most commonly spread from person to person. Bacteria in salivary droplets or in nasal discharge can spread when you cough or sneeze. You could inhale these respiratory droplets directly. These droplets can also settle on surfaces. If you touch your mouth, nose, or eyes after touching something with these droplets on it, you could become infected.

The bacteria is less commonly transmitted through food or water. Since you are unlikely to get strep from animals, there is no need to worry about your family pets.

Incubation Period, Contagious Period, and Duration of Illness

The typical incubation period for strep throat is two to five days. This means, on average, it takes three days from the time you are exposed to the bacteria to the time you develop symptoms.

If you know you have had contact with someone who has been diagnosed with strep throat, be on the lookout for symptoms during the next few days.

Strep throat usually lasts three to seven days with or without treatment. If you are treated with antibiotics, your symptoms are likely to improve within a day or two and you are not considered infectious 24 hours after your first dose.

Left untreated, however, you could be contagious from the time you are exposed to the bacteria until your symptoms resolve. Some resources claim that infectivity could last as long as a week afterward.

Active Infection vs. Carrier State

Not all S. pyogenes bacteria lead to an active infection. Some people live with the bacteria in their pharynx and nasal passages and don't develop symptoms. These bacterial strains tend to be less virulent (harmful to the body). These people are said to be colonized with the bacteria and are carriers of the disease. As many as 20% of school-aged children fall into this group.

Carriers are less likely to spread disease. It remains controversial whether they should be treated with antibiotics to eradicate the bacteria given the small chance that they could infect others. This may be a reasonable option if the carrier has frequent close contact with someone who has a weak immune system (e.g., someone on chemotherapy). It may also be a consideration if there are recurrent infections to other people within the same household.

Lifestyle Risk Factors

Race and gender do not predispose you to infection, but there are other factors that increase the odds of getting strep throat.


Strep throat is most common in children 5 to 15 years old. Younger children can also get infected, but less frequently and often with atypical (not standard) symptoms.

A meta-analysis of 29 articles in Pediatrics showed that among children of all ages who presented with a sore throat, 37% were diagnosed with S. pyogenes, but that prevalence decreased to only 24% for children younger than 5 years old.

Adults are infected at a much lower rate of 5 to 10%. Regardless of age, strep throat is usually treated with antibiotics.

Close Contact

Close quarters make it more likely that infection will spread from person to person. Schools and daycare centers are notorious for this. People who live with someone who has strep throat are also at higher risk of infection.


Hygiene is a common culprit when it comes to the spread of strep infections. Children may cough into their hands or rub their noses without using tissues. Research shows that S. pyogenes can live on the hands for up to three hours.

Hand washing is key to decreasing the spread of infection. When soap and water are not available, consider alcohol-based hand sanitizers.

Also avoid sharing food, drinks, or utensils and kissing during an infection.

Pollution or Smoke Exposure

Whether you smoke or are exposed to second-hand smoke, your throat and airways are likely to be irritated by the particulate matter. This leaves the throat prone to infection from strep and viruses. Air pollution can do the same.

Time of Year

Strep throat can occur year-round, but it has seasonal variations. Infections are more common in late winter and early spring.

Frequently Asked Questions

  • What causes frequent strep throat?

    Genetics may be one reason, according to a 2019 study. Researchers found that kids with recurring strep throat tended to have smaller germinal centers in their tonsils, which usually recognize and fight infections. They also were more likely to have family members who had tonsillectomies. Other causes of recurrent strep throat include having a weakened immune system, not finishing your course of antibiotic treatment, and not replacing your toothbrush.

  • Can babies and toddlers get strep throat?

    It's possible, but not likely. Strep throat is rare in children younger than 3 years old. Contact your healthcare provider if you are concerned that your child may have strep throat. Some symptoms that develop in young children include fever, fussiness, runny nose, and decreased appetite.

8 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Kalra MG, Higgins KE, Perez ED. Common Questions About Streptococcal Pharyngitis. Am Fam Physician. 2016;94(1):24-31.

  2. Centers for Disease Control and Prevention. Strep Throat

  3. Shulman ST, Bisno AL, Clegg HW, et al. Clinical practice guideline for the diagnosis and management of group A streptococcal pharyngitis: 2012 update by the Infectious Diseases Society of America. Clin Infect Dis. 2012;55(10):e86-102. doi:10.1093/cid/cis629

  4. Centers for Disease Control and Prevention. Pharyngitis (Strep Throat)

  5. Centers for Disease Control and Prevention.Strep Throat: All You Need to Know

  6. Marks LR, Reddinger RM, Hakansson AP. Biofilm formation enhances fomite survival of Streptococcus pneumoniae and Streptococcus pyogenes. Infect Immun. 2014;82(3):1141-6. doi:10.1128/IAI.01310-13

  7. Dan J, Havenar-Daughton C, Kendric K, et al. Recurrent group A Streptococcus tonsillitis is an immunosusceptibility disease involving antibody deficiency and aberrant TFHcellsSci Transl Med. 2019;11(478):eaau3776. doi:10.1126/scitranslmed.aau3776

  8. Cleveland Clinic. Why do doctors avoid strep tests for kids under age 3?

Additional Reading

By Tanya Feke, MD
Tanya Feke, MD, is a board-certified family physician, patient advocate and best-selling author of "Medicare Essentials: A Physician Insider Explains the Fine Print."