Giving someone positive energy can definitely help their health, although I don't know if it could go as far as preventing an infection - most of my PTs do "bioenergy work" on me and my body responds to it extremely well, it's not a placebo affect because I never know when they're doing it. Personally, I don't think prayer would work the same - when you are giving someone good energy it's a conscious and interactive experience, the source of "goodness" is YOU - when you pray you're asking a god to send "goodness" to someone else, and at a distance - they are in no way the same thing, as one is active and the other passive.