How salmonella gets into peanut butter, from Scientific American:
http://www.sciam.com/article.cfm?id=salmonella-poisoning-peanut-butter
Contamination occurred after the roasting process, thus answers:
1) animal feces (rats in the vat?)
2) roof leak (ie. birds contaminate leaking water)
If it had been from human bathroom habits, most likely the contaminant would have been E.coli!