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Strep throat is a contagious 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 adenoid (tissue mass located behind the nose in the upper throat) may also become swollen and infected. Left untreated, strep throat can damage the heart valves (in rheumatic fever) and kidneys (glomerulonephritis).

The bacteria that cause strep throat are from a class of bacteria called group A streptococci (strep), also known as Streptococcus pyogenes or group A beta-hemolytic streptococcus. 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 by 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.

There are many different strains of strep bacteria, which can cause a number of different types of illnesses. Thus, even though people develop antibodies to the particular form of strep with which they were infected, they remain vulnerable to other strains. For this reason, people are often diagnosed with strep throat multiple times.

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. These include tonsillitis, sinusitis and otitis media. In some cases, these infections may accompany strep throat. The strep bacteria can also cause bacterial pneumonia.

• Skin infections. The strep bacteria can infect the skin, causing impetigo, cellulitis and erysipelas (inflammation in upper layers of the skin). The U.S. 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 people, cellulitis infection may develop in the rectum (streptococcal perianal cellulitis) or vagina (streptococcal vaginitis).
  
  
• Scarlet fever. Infectious disease that may occur 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.