CMV is a virus that can cause serious infections in patients with suppressed immune systems. It usually affects the lungs and can cause a very serious pneumonia, but it can also affect the intestinal tract, the liver and the eyes. Approximately 2/3 of normal people harbor this virus in their body. In healthy people CMV rarely causes any problems because the immune system can keep it under control. If the patient and/or their donor is positive for CMV, they are at risk of developing CMV disease while the patients immune system is weak post transplant. Usually, this risk is highest during the first 3-4 months after the transplant.