CMV pneumonitis causes

Causes
CMV pneumonia is caused by a member of a group of herpes-type viruses. Infection with CMV is very common. Most humans are exposed to CMV in their lifetime, but typically only individuals with weakened immune systems become ill from CMV infection

Usually CMV produces no symptoms, but serious CMV infections can occur in people with weakened immune systems from conditions such as:

In people who have had organ and bone marrow transplants, the risk of infection is greatest 5 - 13 weeks after the transplant.
 * AIDS.
 * Bone marrow transplant.
 * Organ transplant.
 * Chemotherapy or other treatments that suppress the immune system.