C. diff infections are NOT always caused by antibiotic "overdose," which isn't really a thing - there's dose related toxicity for some classes of antibiotics, but not overdose. In the healthcare setting, infections are usually caused by overgrowth in the setting of depleted colonic flora, but symptomatic infections can also happen with contact with a patient already experiencing a C. diff infection - two years ago, a resident at my hospital ended up with such a severe infection she picked up from a patient that she nearly ended up with a total colectomy. In the community setting, about 50% of cases occurred without any prior antibiotic exposure in the past 30-90 days, at least according to a recent meta-analysis [http://www.ncbi.nlm.nih.gov/pubmed/25058469].