Genetic in our families case.
As for being too clean, its not that . to understand the hygiene theory you need to grasp what you need to do to protect your kids from allergies.
basically expose them to animals and their waste. cows are particularly good.
But, there is a catch, have close exposure to cows and their poop BEFORE you get preggers, and during and after birth take infant near cows and their poop to fully get the protective impact to stop allergies developing.
oh, and bear in mind not all farm animals give that good a protection, asthma is pretty high in pig farmers. this may of course be the result of us living closely with pigs during the middle ages. or when early farming started.
so its not a case of keeping your kids or home overly clean.
As the mother of four, and two with severe allergies i get rather fed up of being blamed for being too clean with my filthy brood.
Exclusion during pregnancy and in early years has been proved to cause some food allergies, peanut in particular. certainly the old advice to avoid during pregnancy lead to a huge increase in peanut allergy in non atopic families.
If you are wondering if your kids are likely to develop allergies, if you the mother have atopic diseases, asthma, hay fever, eczema risk higher. if its just dad, its a bit lower. both well... more likely.
But take heart, both me and hubby have atopic disease and only 2 out of my 4 live with it.
I often hear the equally annoying rubbish about 'chemicals' or 'vaccination' being a cause. Crap in my view and even if the latter was slightly likely am so glad my kids are protected from diseases that would have severely damaged or killed them. I have family members who live with damage from such diseases, like rubella and whooping cough. All of which are far worse imo than severe, life threatening food allergy.