Group A Streptococcus (GAS) or Streptococcus pyogenes is a bacterium commonly found in the throat and on the skin. Many people can carry this bacterium without any symptoms of disease. The majority of group A streptococcal diseases are relatively mild illnesses, such as streptococcal pharyngitis ("strep throat") or impetigo. However, occasionally these bacteria can cause potentially life-threatening "invasive" infections. Invasive infection occurs when group A Streptococci invade parts of the body where bacteria are not normally present, including the blood and soft tissues such as muscle or fat.