Strep throat is caused by streptococcal (strep) bacteria, most often by group A beta-hemolytic streptococcus (GABS). Other types of strep that can sometimes infect the throat are groups C and G strep bacteria.

A strep infection causes the throat (pharynx) and the tonsils or adenoids to become irritated, inflamed, and painful.

Sore throats are most commonly caused by viral infections or other irritants such as smoke, allergies, dry air, or a throat injury, and not by a strep infection. For more information, see the topic Sore Throat.

How the strep infection is spread

Strep throat can be passed from person to person. When a person infected with strep throat breathes, coughs, or sneezes, tiny droplets containing the strep bacteria are released into the air and are breathed in by other people.