CMV can also cause serious infections in patients with weak or suppressed immune systems. It usually affects the lungs, causing a very serious pneumonia, but it can also affect the gut, 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, but after a transplant, the risk of developing CMV disease is much higher because the immune system is so weak.