I am a sensitve person,so if i started puking because everyone else was puking in the church would that make me possessed.
I believe that a demon cannot posses a christian,they can influence you to sin by way of suggestion, visually. A person can expose themselves by way of drinking,drugs or even when meditating like the Maharishi. When being around these people,they seem like the walking dead,that is probably the closest thing to a ghost i have seen. Pale and incoherent.