Infections in pregnancy present a particular problem. The pregnant woman's immune system has to adapt to what is essentially a foreign substance in her midst: her fetus. Because of this, exactly how she responds to invading viruses, bacteria and parasites may not be the same as how she responds when not pregnant.
Generally we think of our responses to infection in pregnancy as still pretty good, so they say that this is one of the "paradoxes of pregnancy." But, it may not always be so, and infections that are so mild as to not be on anyone's radar may suddenly become very interesting when they occur in a pregnant woman. We heard about this with swine flu, and infections like malaria. With malaria even women who are supposed to be immune because of prior infections can suddenly become susceptible again if they get pregnant!
So it is with the infection from a food born bacteria called listeria, one such disease that can be quite important if you are a pregnant woman. Fortunately it's so rare! And so hard to diagnose! And more common in pregnant patients than in the general population! A 2008 MediReview article said about 20 times more common.
The actual disease is Listeriosis. It is due to the bacteria Listeria monocytogenes. We fight this bacteria off with so-called cell-mediated immunity, which is a bit less functional in the pregnant patient. When you get sick, most only seem to be "sort of flu-like," but severe illnesses that progress all the way to seizures, brain infections and heart infections have been reported.
The fetus could get infected, and only blood cultures from the mom or amniotic cultures from the uterus can really prove a case. Most outbreaks are from contaminated foods such as deli meats or non-pasteurized cheeses. And so this is where it is very important not to overeat these sorts of foods. Always wash your counters well if you've been preparing those sorts of foods on them, and if you develop odd symptoms or have a persistent fever, call your gyno!