Listeriosis infection, which usually doesn't cause anything more harmful than a gastro-intestinal flu, can lead to miscarriage or serious illness for the unborn child during pregnancy.
Listeriosis bacteria are found especially in products of raw milk (not pasteurised), and in raw meet and fish. It is best during pregnancy to eat only pasteurised milk products and avoid raw meat and fish. Salads with wheat germ and sprouts can also contain listeriosis bacteria and thus have to be very thoroughly rinsed.
Listeriosis infection in pregnant women is still very rare, though there have been some cases in the last few years (two infections per 1000 pregnancies).