Strep throat is caused by Group A Streptococcus bacteria. It is the most common bacterial infection of the throat.

Strep throat is most common in children between the ages of 5 and 15, although it can happen in younger children and adults. Children younger than 3 can get strep infections, but these usually don't affect the throat.

Strep throat is most common in the late fall, winter, and early spring. The infection is spread by person-to-person contact with nasal secretions or saliva, often among family or household members.

There are many strains of strep. Some strains can lead to a scarlet fever rash. This rash is thought to be an allergic reaction to toxins made by the strep germ. On rare occasions, strep throat can lead to rheumatic fever if it is not treated. Strep throat may also cause a rare kidney complication.