A wide range of organisms are capable of infecting the developing fetus. They range in size from the smallest viruses, e.g. parvoviruses, through to bacteria such as Treponema pallidum and protozoal parasites, namely Toxoplasma gondii. Many of these agents are, or once were common childhood infections, but if infection is delayed until adulthood, the fetus may be put at risk. Vaccination has almost eliminated the burden of disease in the case of rubella and hepatitis B, but significant problems remain with CMV and parvovirus, where neither effective vaccines nor safe anti-virals are yet available to prevent fetal infection and/or pregnancy loss.