The human cytomegalovirus (CMV) is common and usually causes no symptoms in healthy individuals. However, CMV infections can be life-threatening in individuals with improperly functioning or immature immune systems, such as fetuses. Women can become infected with CMV for the first time (primary infection) during pregnancy. If CMV is transmitted from mother to fetus before the second trimester, the infant can suffer from severe disorders such as hearing loss and delayed development.