streptococcal pharyngitis or strep throat is an illness that is caused by the bacteria called “group a streptococcus”. strep throat affects the throat and the tonsils. the tonsils are the two glands in the throat at the back of the mouth. strep throat can also affect the voice box (larynx). common symptoms include fever, throat pain (also called a sore throat), and swollen glands (called lymph nodes) in the neck. strep throat causes 37% of sore throats among children.