Strep throat is a bacterial infection of the throat and tonsils caused by Streptococcus pyogenes (group A streptococcus). It commonly results in sudden onset of a sore throat, pain when swallowing, fever, red and swollen tonsils sometimes with white patches or streaks of pus, and swollen lymph nodes in the neck. Other symptoms can include headache, nausea, or fatigue. Strep throat is highly contagious and spreads through respiratory droplets from coughing, sneezing, or close contact with an infected person. If left untreated, it can lead to complications such as rheumatic fever or kidney inflammation. Early recognition is important to reduce the risk of spread and complications.