His mother Carole was surprised and alarmed when she discovered just how careful she would have to be.
But somehow most sufferers get by through a combination of lifestyle changes, learning new skills and - in some cases - extreme caution.
For those who do not know what is going on, the effect of an allergic reaction can be terrifying and confusion.
In a new two-part series, the BBC followed seven sufferers as they found out about their allergies and what they can do about them.
Slightest contact
In the first episode of Allergy Alert, to be broadcast on Tuesday, viewers meet Thomas Donelly - a three-year-old who is allergic to nuts and has almost died from his condition.
His mother Carole was surprised and alarmed when she discovered just how careful she would have to be.
"I came home from work one day and kissed him hello," she said.
"Within minutes his face was blotchy and his lips had swollen up. We realised that I'd eaten peanuts some six hours earlier and what I'd eaten affected him as well.
"It was very frightening because obviously at that age I had total control over what he was eating but you can't control contact with other people."
It was at that stage Carole decided to stop Thomas's elder brother Charles eating peanuts as well.
Extreme reaction
Geoff Rutter - who confesses to enjoying eating just about anything - was shocked to discover he was allergic to snails and seafood.
The allergy sneaked up on him at an office party.
"I went to the table and started eating snails and got to the last one and started feeling a bit fuzzy," he said.
"I got up and started walking across the restaurant floor and the next thing I know I'm waking up in a chair being violently sick."
He was told he had been unconscious for seven minutes, but remembered none of it.
Geoff had suffered the most extreme reaction, going into anaphylactic shock.
Body's mistake
Like all allergic reactions, this occurs when the body mistakes a usually harmless substance as a threat and launches an immune attack on it - causing the body to release histamines.
In the case of anaphylactic shock, this is so great that the victim quickly suffers muscle contractions and swelling, often closing the throat, making it difficult to breathe.
Sufferers may also experience abdominal pain, vomiting and diarrhoea, and in extreme cases the shock can lead to death.
Considering the potential threat to his life, Geoff was keen to find out what he can and cannot eat.
Allergy clinics establish this using a skin-prick test, which measures how skin reacts to the foods under suspicion.
As it turned out, Geoff was disappointed to find that snails are a definite no-no while he should probably avoid most seafood.
Nuts are out
For Thomas Donelly, the skin test is used to refine what his parents already know - to find out exactly which nuts cause the reaction.
Peanuts and almonds are identified as the two to be avoided, and his parents learn how to use an adrenaline pen in the event of him suffering another reaction.
Adrenaline is the most effective way to counter the effects of an extreme reaction.
Carole found the allergy clinic visits had paid off.
"We've got the information we need now, and hopefully that will keep him safe."
You can find out more in Allergy Alert on BBC One on Tuesday and Thursday at 20.30 BST, 19.30GMT. There is a BBC Website to accompany the series, which provides more details of allergies, treatments and where help can be found.