Cross-contact is the term for allergen protein transferring from one food to another. Nobody meant to put it there; a pan, a board, a spoon, a pair of hands or a shared fryer moved it. The dish now contains an allergen that appears on nobody's recipe.
Equipment does most of it. A knife that cut a sandwich, a toaster that has held bread, a grill that has held fish, a fryer that has cooked something battered — each of them carries protein into the next thing that touches it.
So do hands. Flour dust settles on everything in a bakery, and crumbs travel further than anyone expects. Washing with soap and water is what shifts allergen protein off hands and surfaces — it is the thing that works, which is not the same as a promise that nothing is left. Hand sanitiser is aimed at germs and does not do the same job.
A buffet is a category of its own. Serving spoons move between dishes, tongs get swapped, and a child at eye level with the counter reaches for whatever is nearest. None of that is anybody's fault; a buffet is simply a room designed for mixing.
Cooking oil gets reused, so a fryer that has cooked one thing is carrying it into the next. Steam and airborne protein matter less often than people fear, but not never, and there are foods and rooms where they matter a great deal. Which of those a particular person is in is a question for them and their doctor, not for this page.
Precautionary labelling — the may contain wording, the made in a factory that also handles wording — is where cross-contact meets the label. In most countries that wording is voluntary and unmeasured, which means its absence is not a promise and its presence is not a measurement.
That leaves families making judgement calls on incomplete information, which is exhausting and is not a failure of anybody's nerve. A kitchen that says plainly what it cannot guarantee is doing far more for them than one that reaches for warm words about a shared pan nobody has looked at.
None of this is a set of rules for a particular household or a particular kitchen. It describes how cross-contact happens, and nothing more. What is workable in one home, one school or one restaurant is a conversation between the people in it and the allergic person's own doctor.
Gloves deserve a mention because they are so often mistaken for a solution. A glove that has touched an allergen carries exactly as much protein as a hand that has, so changing gloves matters far more than wearing them.
Questions this page raises
Ezzie will have a go at any of these.