One of my wife's friends got persistently sick last year. She just could not get better. Sometimes she'd be fine for a week or two, but then she'd get sick again. Eventually it came down to her needing to document everything she did each day - and they discovered she was getting sick from warm butter.

Turns out her mom had come over at some point and saw that she refrigerated butter and said "you don't need to do that, it's so much easier to use when warm and it doesn't go bad." Yeah, that's the case if you eat a stick of butter in a few short days. But you can't leave it out for more than that or it starts getting filled with all sorts of germs.