Someone with a severe nut allergy may die as a consequence of someone else munching nuts in an enclosed space that there is no easy escaping from once it's at cruise altitude.

Why are there more nut allergies these days is an interesting question, but it may have to do with not quiet as many severe allergy sufferers dying in childhood.

In any case, one should take these things serious - the young lady dying of an allergic reaction to a pret sandwich on a BA flight a few months back is a sad example as to why this is important.