I can add that last year I was adopted by a purebred kitten, a 15 week
old Somali.  He was perfectly healthy, no parasites, all the shots,
from a very loving and respected breeder (not a kitten mill) and had
not even had the problem with overbloom (the change from the Mother's
parasites to his own that many kittens are troubled with.) Three weeks
after he came here he came down with smelly farts and poops.  Turns
out he had caught Giardiasis from Lady, my black cat, whom I'd had 10
years and had shown NO SYMPTOMS of colonitis.  So both cats benefited
from the treatment with Fenbendazole.  How did Lady herself get
Giardiasis? probably from a previous, deceased, cat, who had himself
had some form of bowel disease, never fully identified.  The mode of
infection for parasites is usually through a shared litter pan.  And
yes Giardidasis is the same in cats and Humans and if one is very
careless there can be cross-infection.

As a rule no cat food will cause smelly do-dos in a healthy cat except
maybe for a couple of days after a radical change in diet.

..  Steve  ..