Thankfully, I have no known allergies of any kind. Up until seven years ago, I never gave that a second thought. After meeting my girlfriend, I'm reminded every day just how lucky I am.
She has more food allergies than I could possibly list. The "short" list: tomato, potato, garlic, corn, nearly every herb and spice known to man, every type of yeast we can test, alcohol (and any fermented product we've tried), most fish and shellfish, peanuts, walnuts, almonds, and nearly all other nuts... Apples, bananas, every common berry, avocado... the list goes on and on.
Furthermore, a lot of people are lucky in that they don't necessarily have problems with trace amounts of the problem proteins contaminating other products through shared manufacturing. She gets a reaction, which varies in severity by cause, seemingly no matter how small the residual.
Thankfully, most of her allergies "only" cause severe discomfort, elevated heart rate and nausea/vomiting and not the severe swelling of the throat common with anaphylaxis. So it's rarely life threatening, just very ****ty.
So how do we deal with it? She hasn't had a meal that wasn't cooked by one of us in over five years. Through much trial and error, we've found products that are completely free of her allergens. Any time we're interested in a new product, we contact the manufacturer directly to ask about shared equipment at their facilities to limit the risks. And we've learned the hard way that we need to stay diligent even with products that have been safe in the past, as ingredients and manufacturing/processing can change without warning.
And so any time we're away from home for an extended period, we bring her "safe" food and bottled water. When we go on vacation, we stay at places that have a kitchen and bring our own cookware and ingredients. A few times this wasn't an option, I rented a commercial kitchen for a few hours to prepare meals. Once, I talked the chef at the hotel into letting me use a small area of their kitchen.
As someone who loves cooking, it was very difficult at first. I made a lot of mistakes. But over time, I've gotten used to it and have found what works and what doesn't when making substitutions. It's something we both have to be constantly aware of and plan around.