If you are cooking for someone with a food allergy you need to be very careful when you are preparing food.  Common food allergies include:-

Eggs
Fish
Gluten
Dairy e.g. milk
Crustaceans
Nuts
Peanuts
Mustard
Lupin
Molluscs

If you are cooking for someone for the first time contact them to make sure they don't have any food allergies.  Not only do you need to be careful of what ingredients you use, you also need to make sure ingredients are kept seperate from others to avoid cross contamination.  Make sure dishes you cook in are not contaminated and be careful of any sauces that you may add to a dish or have on the table as these may also contain food people are allergic to.

Keep a copy of the ingredients you use and any food labels so you can show your guest exactly what is in the dish they are eating. They are bound to be cautious knowing that if they eat certain foods it can make them very unwell and even be life threatening.