I have an allergy? sensitivity? intolerence? of all things cheesy e.g. creme fraich, yogurt, sour milk and any kind of cheese. I am however fine with fresh cream, milk, butter etc. Apparently it is the bacteria that 'turn' the milk that causes the problem. I am fine when I prepare and eat food at home but have suffered severe reactions (horrendous stomach pain and the runs for 2 or 3 days) when eating out. It only takes the smallest amount e.g. grated cheese stuck to the bottom of a ham roll prepared on the same board, a swirl of creme fraiche in soup, to trigger an attack.
As this seems to be comparitively rare in the UK (though common in Italy) there are never any warnings such as you get for nuts, though I can avoid problems by choosing dairy free dishes.