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Strep Throat Causes: What Causes Strep and How It Spreads

Strep throat is caused by bacteria, not by a virus. Knowing where the infection comes from, how it passes from person to person, and who is most likely to catch it can help you protect your family and recognize when to get tested.

This page is educational and is not a diagnosis.

Table of Contents

What causes strep throat?

Strep throat is an infection of the throat and tonsils caused by group A Streptococcus bacteria, often called group A strep. These bacteria are contagious.

Most sore throats are caused by viruses, such as those that cause colds. Those sore throats are not strep and do not improve with antibiotics. Strep throat is the bacterial exception, which is why doctors test for it. See strep vs viral sore throat.

How strep throat spreads

According to the CDC, group A strep spreads when people:

  • Breathe in droplets from someone who is infected and is talking, coughing or sneezing
  • Share a plate, utensils or a glass with someone who has the bacteria
  • Touch infected skin sores caused by group A strep

Some people carry group A strep without having symptoms or seeming sick, and they can still spread the bacteria to others.

How soon symptoms start

It usually takes 2 to 5 days after exposure for someone to become ill with strep throat. A child who was exposed at school, for example, may start feeling sick a few days later.

Who is most at risk?

  • Children 5 through 15 years old get strep throat most often, but anyone can get it.
  • Young children under 3 rarely get strep throat.
  • Crowded settings increase the risk, including daycare, schools, detention facilities, homeless shelters and military training facilities.
  • Adults get it too. The CDC estimates that group A strep causes 5% to 15% of sore throats in adults, compared with 20% to 30% in children.

Why some people keep getting strep

Repeated strep infections can happen after new exposures, for example when the bacteria keep passing between people at home or school. If you or your child gets strep often, talk with a healthcare provider. Read more in why you keep getting strep throat and recurrent strep throat causes.

How to lower your risk

The CDC recommends these steps to prevent group A strep from spreading:

  • Wash your hands often with soap and water.
  • Cover coughs and sneezes.
  • Don’t share cups, utensils or bites of food with people who are sick.
  • Wash cups, utensils and plates after someone who is sick uses them.
  • Take antibiotics as prescribed if you are diagnosed with strep. Antibiotics treat the infection and help prevent serious complications.

The American Academy of Pediatrics advises that children with strep throat can return to school or child care once they have had two doses of an appropriate antibiotic 12 hours apart and no longer have a fever. See how long strep throat is contagious and preventing strep throat in families.

When to see a healthcare provider

Contact a provider if you or your child has a sore throat with fever, pain when swallowing, red and swollen tonsils or tender neck glands, especially without a cough or runny nose. A rapid strep test or throat culture can confirm the diagnosis.

Get medical care right away for trouble breathing, trouble swallowing liquids, drooling, or signs of dehydration.

Medical sources

This information is for education only and does not replace advice from a qualified healthcare provider.