How could I have contracted an E-Coli infection? Im taking Bactrim?

It could be not wiping properly, it could be that someone ELSE is not wiping properly and then not washing hands & spreading it that way, it could be in the dirt from livestock and then if hands aren't washed, etc. Basically it's everywhere, but washing hands regularly usually keeps it from spreading around - there's more than one strain of it, just like a lot of other bacterias. It's when you ingest it via uncooked/undercooked foods that it causes problems. Bactrim is pretty heavy-duty - may get diahrea or stomach upsets from it.