Strep throat (streptococcal pharyngitis)

Strep throat, known medically as streptococcal pharyngitis, is a throat infection caused by bacteria. The pharynx is the medical name for the throat, and the tonsils are the small lumps of tissue you can see at the back of the mouth, toward the sides. This infection affects both the pharynx and the tonsils. The bacterium behind it is called Streptococcus pyogenes, also known as group A strep. Strep throat is more common in children than in adults.

These bacteria usually live in the nose and throat. When someone who is infected talks, coughs or sneezes, they release respiratory droplets, the fine spray that comes out of the mouth, a lot like the mist from a plant sprayer. The bacteria ride in those droplets to another person. Once they settle into the throat, they often leave the tonsils red and swollen and cover them with white patches of pus, the thick fluid that collects where there's an infection.

Still, not every sore throat is strep throat. Most are viral, meaning they're caused by viruses, not bacteria. When a sore throat comes with a cough, a runny nose or hoarseness, that rough, scratchy voice, those signs make this infection less likely. But less likely doesn't mean impossible: they don't rule it out.