Tell the landlord that it smells as if something has died in there, and that you are afraid they have left rotting food, or are letting the dogs poop inside.

That sort of thing is really difficult to clean up. It soaks into the walls.

It very well could be cooking smells. I and my family love onions and cook with them all the time but I swear the after smell... it smells like BO to me. LOL

Have your landlord come sniff. If it is a cooking smell he can't do anything about it. If they don't wash he can't do anything about that either. If it is a pet smell he might be able to do something about that.

I always thought it was usually a rule that only two people per the number of bedrooms in a unit...is your landlord aware how many are occupying the unit? (Federal Housing Authority guideline, but not a law)

Mention the problem to the landlord--its hard to believe that the landlord knows about the pets in such a small apt. Mention that it smells like pet poop and you may get a quick reaction.,

Check with you landlord first.
Then if you live in a decent size city you could check with building and saftey. Many cities have rental inspectors. They will come out for anything in my area. Sewer smells, tenants cooking, etc. I also had a tenant that cooked cultural foods. One evening out of the 6 tenants 3 called. I stopped over found the smell and mentioned others found it disturbing. They were good kind people and we never had another problem.

I"ve had the neighbors with no hygeine recently. It wasn't as bad as the place I live have doors opening outside. But I hated it when we had to ride the public transportation together as I could barely stand to be on it with them.
As a non smoker I have neighbors that smell up the area with cigarette smoke and throw their butts anywhere they please.