First off, I need to mention that many of my rh negative members take offense to the term "disease" and rather consider the reaction of a mother's body natural protection from the rhesus factor.
Rh disease (also known as Rhesus isoimmunisation, Rh (D) disease, Rhesus incompatibility, Rhesus disease, RhD Hemolytic Disease of the Newborn, Rhesus D Hemolytic Disease of the Newborn or RhD HDN) is one of the causes of hemolytic disease of the newborn (HDN). The disease ranges from mild to severe, and typically occurs only in some second or subsequent pregnancies of Rh negative women where the fetus's father is Rh positive, leading to a Rh+ pregnancy. During birth, the mother may be exposed to the infant's blood, and this causes the development of antibodies, which may affect the health of subsequent Rh+ pregnancies. In mild cases, the fetus may have mild anaemia with reticulocytosis. In moderate or severe cases the fetus may have a more marked anaemia and erythroblastosis (erythroblastosis fetalis). When the disease is very severe it may cause haemolytic disease of the newborn (HDN), hydrops fetalis, or stillbirth.
Rh disease is generally preventable by treating the mother during pregnancy or soon after delivery with an intramuscular injection of anti-RhD immunoglobulin (Rho(D) immune globulin). The RhD protein is coded for by the RHD gene.