A few things have to be considered when trying to figure out how long your strep throat is contagious. For example, after exposure to strep throat (which is caused by the bacteria streptococcus pyogenes), there is a 2- to 5-day period before you start to experience symptoms. During this time, you will probably unwittingly share your germs with someone else. Once you start to have symptoms and suspect you may have strep throat, you are contagious and should start taking precautions not to infect those around you. If you're not sure whether you have strep, you may wish to read:
If you are diagnosed with strep, you will stop being contagious approximately 24 hours after starting an antibiotic. Antibiotics should help alleviate your symptoms within one or two days, although in some people they may last up to a week. (Without treatment, strep throat generally resolves on its own in about a week.) If you don't have any reduction in fever or other symptoms within 48 hours of starting antibiotics, contact your physician.
Even though your symptoms may have ceased with antibiotics, you should always take your full course of antibiotics as prescribed, primarily to avoid rare, serious complications, like rheumatic heart disease. Treatment with antibiotics should be started within 9 days of symptoms to reduce your risk for complications of strep throat.
Medline Plus. Strep Throat. Accessed: April 9, 2011 from http://www.nlm.nih.gov/medlineplus/ency/article/000639.htm
The Merck Manuals Online Medical Library. Streptococcal Infections. Accessed: April 9, 2011 from http://www.merckmanuals.com/home/sec17/ch190/ch190u.html#sec17-ch190-ch190s-310