I've heard this before many times that food that was left under ones bed that now has on it "ruach rah" can be "purified" by tovelling the food in the mikveh (perhaps someone can help with a source ...
In general (DeOraisa), when Tumah spreads it lowers itself by a level. Moreover, a Keli can't make a Keli Tamei, nor can a food item make another food item Tamei (at least by Chullin). So a ...
If according to my Bible translation - please correct me if it is wrong - anything a woman in niddah or a state of tumah, or 'impurity', touches or sits on becomes 'impure', can food cooked by her be ...