The purpose of this study is to determine the optimal outpatient treatment strategy of uncomplicated skin and soft tissue infection (SSTI) in areas of the United States where the prevalence of Community-Acquired Methicillin-Resistant Staphylococcus (S.) aureus (CA-MRSA) is high. Infection with the S. aureus bacteria that is resistant to antibiotics is a cause of SSTIs. Three oral antibiotics will be tested for off patent treatment. Patients will receive Trimethoprim/Sulfamethoxazole (TMP/SMX), placebo (substance containing no medication), clindamycin, or cephalexin or some combination of these. The study population will include 2,235 volunteers, children 13 years of age and over and adults presenting to 5 large urban Emergency Departments. Therapy for acute uncomplicated SSTIs, including abscess, infected wound, and cellulitis will start on the day of enrollment. Participants may be involved in study related procedures for about 9 weeks.