Strep throat is inflammation and swelling of the throat, tonsils and lymph nodes. Unlike sore throat, which is caused by infection with viruses, strep throat is caused by infection with bacteria. While most sore throats generate mild symptoms and clear naturally, without the aid of medical treatments, strep throat generally requires a course of antibiotics. Untreated strep throat can lead to serious complications such as otitis, heart problems, renal problems and even rheumatic fever. On the premises of a weak immune system, strep throat can be very severe, in which case doctors recommend prompt medical treatment with antibiotics.