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Strep throat is a bacterial infection that causes inflammation within the throat. It affects the pharynx (back of the throat) and tonsils (two oval-shaped tissue masses located inside the mouth, at either side of the throat). Sometimes, the adenoids (tissue mass located behind the nose in the upper throat) may also become swollen and infected.

The bacteria that cause strep throat are from a class of bacteria called group A streptococci (strep). Most often, a person develops strep throat after exposure to a person already infected with the bacteria. Other times, the bacteria can live harmlessly in the nose or throat for some time without causing infection, and can then be triggered due to stress or immune system dysfunction, resulting in infection. 

Strep throat occurs most often in children between 5 years and 15 years old, although it may affect people of all ages. Approximately 15 percent of sore throats in children are due to strep throat, according to the American Academy of Family Physicians.

Untreated strep throat can spread to other areas of the body and lead to serious medical conditions. However, these complications are rare in the United States, since prompt and effective treatment is delivered in most cases of strep throat. These complications include:

• Rheumatic fever. A severe inflammatory disease that causes joint pain and may cause permanent heart problems. Rheumatic fever occurs approximately 18 days after an episode of strep throat, according to the National Institutes of Health (NIH). It most often affects children.
  
  
• Glomerulonephritis. Inflammation of the kidneys that may lead to kidney failure. This condition may occur after an untreated case of strep throat, although it occurs more often after a strep infection of the skin.
  
  
• Sydenham’s chorea. Sometimes follows rheumatic fever. This disorder involves uncontrolled, jerking movements of the muscles in the torso, arms and legs.

In rare cases, strep throat can result in the development of obsessive-compulsive disorder (OCD) in some children. This mental health condition is characterized by recurrent, persistent thoughts that patients cannot control (obsessions) and/or an uncontrollable urge to perform certain actions over and over (compulsions). Such cases are known as pediatric autoimmune neuropsychiatric disorders associated with streptococcal infections (PANDAS), and their cause is unknown.

There are many different strains of strep bacteria, which can cause a number of different types of illnesses. In addition to strep throat, strep bacteria can cause inflammation and infection in other areas of the body, such as:

• Ear, nose and throat problems. This includes tonsillitis, sinusitis and otitis media. These infections may accompany strep throat.
  
  

  

• Skin infections. The strep bacteria can also infect the skin, causing impetigo, cellulitis and erysipelas (inflammation in upper layers of the skin). The Centers for Disease Control and Prevention (CDC) estimates that over 10 million cases of strep-caused skin and throat infections occur every year. In some children, cellulitis infection may develop in the rectum (streptococcal perianal cellulitis) or vagina (streptococcal vaginitis).
  
  
• Scarlet fever, an infectious disease that may occur in children following a strep throat infection. It can cause a bright red rash, sore throat and fever.
  
  
• Severe, invasive infections. These include bacteremia (infection that enters the bloodstream), streptococcal toxic shock syndrome (causes blood pressure to drop and vital organs to fail) and necrotizing fasciitis (flesh-eating disease). These infections can be fatal. An estimated 9,000 invasive infections occur annually in the United States, according to the CDC.