A protozoal infection acquired from coming in contact with infected cat feces or more rarely, raw, infected meat. Although this is a mild infection in healthy adults, it can cause serious problems for the fetus. Toxoplasma can cross the placenta and cause miscarriage, compromised fetal growth, or other problems. If you acquire the infection before you become pregnant, there is little to worry about. You will have antibody to the infection, are unlikely to become reinfected and are very unlikely to have an affected child. Blood tests can determine whether you have had toxoplasma infection in the past.
However, if a woman acquires toxoplasma infection for the first time during her pregnancy, her symptoms will still be mild but her unborn infant may be significantly affected. Toxoplasma can severely damage a child's developing nervous system. Unfortunately, this infection may not be obvious at birth. It may appear later as learning disabilities, impaired vision, or developmental delay.
There is treatment for this infection, but it is not nearly as effective as prevention. Pregnant women should never have contact with a cat's litter box. Even after petting your cat, you should probably wash your hands.