How come the mother can be fine with no illness but yet the listeria can harm the fetus still even if the mothers not sick!?

Transmission. Listeriosis is transmitted via contaminated foods. Pregnant women may get symptoms with nausea, vomiting, diarrhea, fever, etc. Many times women can be without symptoms and carry bacteria in their vagina or bowels. Infection to baby occurs via placental infection, rupture of membranes or when baby travels down the birth canal.