My son had, at one time, a "deadly" allergy to fire ant stings to use the allergist's diagnosis. It really gets your attention when a doctor tells you a single little ant could kill your child. After several years of allergy shots it is more manageable now but we still have to keep the epi-pen handy.
I remember reading a message thread on flyertalk a while back where people were terribly cruel in their comments about if they'd be okay if a flight banned peanuts because of a passenger's allergy. It was "screw them", "I'll bring my own peanuts aboard", etc. It is hard to understand I guess, but for some people just being in the same enclosed space with peanuts can cause a problem.