Well if someone is eating their recommended calorie amount in a day, for instance a guy getting 3000 calories, and all of a sudden after coming home from a vacation in the tropics and they start having stomach problems and they start feeling hungry even after 3000 calories in a day, then maybe they should suspect something like parasites.
But if someone is only getting 1000 calories in a day, parasites aren't robbing them of nutrition. Under-eating is.