Start taking notes of times you feel sick to figure out what food it is/when the food is getting spiked. See if the place and time matches up with anyone's patterns . Offer food to hallmates you suspect to see if they reject. If he/she doesn't reject the food and doesn't get sick, said piece of food isn't spiked/it's all in your head. If he/she doesn't reject and gets sick, they didn't do anything but the food is spiked. Another possibility is that he/she knows that you're on to him/her so he/she pretendeds no to know. If he/she rejects the food, don't eat it, add them to your pool of suspects and keep looking for patterns. You can also keep record of the food you eat. Soup may be messed with one way, a sandwich another. This limits the pool of methods and supplies used by the suspect. Ask your GP about your symptoms. If you find that nothing is wrong with your diet and you don't just have a runny nose and a wild imagination, a chemist/doctor etc.. May be able to identify what is making you sick and where the suspect may have acquired it from. There's loads of other advice i could give but it looks like everything has already been said.(; Peace!