Not hard to "accidentally" add crushed nuts to things. I heard some guy's girlfriend killed him by kissing him after eating peanuts and he had an allergic reaction.
I could add a small amount of powdered peanuts to several things you are likely to consume during the day, not enough for anything but chemical analysis to uncover, but enough to gradually build up and kill you. I'm taking wild guesses here on things you are likely to eat, but I could mix it with things like Milk, Tomato sauce (Ketchup), Soft Drink (Soda, like Pepsi etc.), into Cereal, easily into anything powdered you might consume like Milo or nutritional supplements, sprinkle it over bread. Depending on the nature of the allergy, I could even put it into any central heating or ventilation in your house... The gradual build-up of it acting like pollen and hay fever.
It would be genius if a) I actually wanted to kill you and b) I didn't post it publicly on an internet forum
