Group A Streptococcus bacteria causes Strep Throat. Since it is a bacterial infection therefore you need antibiotics for treatment. Without antibiotics it lasts for 2 weeks whereas with antibiotics it lasts for 3-5 days. The symptoms begin to improve from day 1 of antibiotic use. Most important to understand is that you must complete the course of antibiotics instead of leaving it in the mid. Doing so makes bacteria resistant to antibiotics. Lack of adherance to antibiotics is a growing medical concern.