There are several foods that should be avoided before and during pregnancy because pregnancy affects a woman's immune system. A pregnant woman and her unborn baby are more susceptible to the bacteria, viruses, and parasites that cause foodborne illness. Even if she doesn’t feel sick, some “bugs” like Listeria and Toxoplasma can infect the baby and cause serious health problems. A baby is also sensitive to toxins from the food that you eat, such as mercury in certain kinds of fish.