Has a contraindication (sensitivity or allergy) to trimethoprim/sulfamethoxazole and ampicillin. The patient has a contraindication (e.g. sensitivity/allergy) to both trimethoprim/sulfamethoxazole and ampicillin Has a contraindication to administration of amoxicillin, ampicillin, ciprofloxacin, erythromycin, gentamycin, penicillin, trimethoprim/sulfamethoxazole, and vancomycin Has a contraindication to administration of ampicillin or trimethoprim/ sulfamethoxazole. Known allergy to both penicillin and trimethoprim/sulfamethoxazole. Participants who are allergic to only one of these antibiotics are allowed to enroll Has a contraindication to administration of trimethoprim/sulfamethoxazole or ampicillin. Has a contraindication to administration of trimethoprim/sulfamethoxazole or ampicillin. Patients with a contraindication (e.g. sensitivity/allergy) to trimethoprim/sulfamethoxazole or ampicillin Infected at time of protocol entry, or receiving antibiotics (other than prophylactic trimethoprim sulfamethoxazole) Known allergy to both penicillin and trimethoprim/sulfamethoxazole. Participants who are allergic to only one of these antibiotics are allowed to enroll On prophylactic antibiotics, such as trimethoprim-sulfamethoxazole for pneumocystis prophylaxis or post-transplant penicillin prophylaxis