Strep throat (an infection of the throat caused by group A streptococcus bacteria, also called Streptococcus pyogenes) is most contagious starting a few days before symptoms show up and decreases after a person has been on antibiotics for at least 24 hours.aused by group A strep infection, the illness causes fever, sore throat, white spots on the tonsils, swollen lymph nodes, a bright-red strawberry tongue, and a tell-tale red rash that starts on the abdomen and spreads throughout the body within two days.