All foods can be bad...

 Dr. Kurt Harris of PaNu dispenses his wisdom:

I find the argument that cow's milk, which has much similarity to human milk, is a poisonous alien substance, yet the proteins found in the muscles and organs of the cow are the nectar of the gods a little inconsistent. I will once again point out that a fair number of those with casein allergy are also allergic to bovine serum albumin (BSA) in the muscle of beef. Yet no one suggests that beef is not paleo because some folks with a history of eating the SAD get a rash when they eat it. Same thing with shellfish. Hell, some people go into anaphylactic shock with prawns, even though hominids have eaten seafood for a million years or so. Beware the seductive logic of paleolithic food re-eanctment. PaNu is not about historical re-enactmant - it is about health informed by many lines of evidence and reasoning

That's why I made the paleo foods section on this site. Eating paleo is about a philosophy of eating, not about branding some foods as bad and others as good. All foods have the potential to cause people problems. Allergies seem to be malfunctions of the immune system and seem to have no baring on whether the food is appropriate for humans or not. Luckily, allergies are fairly easy to identify. The ones that are hard are foods that contain substances that cause chronic low-grade problems that may or may not turn into a full blown illness much later. These foods including sugar, gluten, and unfermented soy, are what the paleo diet identifies and really encourages completely eliminating.