(HealthDay News) -- Strep throat is most common in children aged 5 to 15, but people younger or older get it, too.

Most cases of sore throat are caused by viruses, and no antibiotic treatment is needed. But strep is caused by Streptococcus bacteria. If it's confirmed by tests including a throat swab, antibiotics should be prescribed, the U.S. National Library of Medicine (NLM) says.

Symptoms of strep vary. Some people have only a mild sore throat. But others can get more severe forms of strep that cause a scarlet fever rash or can lead to rheumatic fever if left untreated.

Strep usually starts with a high fever that spikes the second day after exposure. Symptoms of severe strep may include nausea, neck pain and swollen lymph nodes.

Most people with strep are contagious until they have been on antibiotics for 24 to 48 hours, the NLM says.