Cytomegalovirus, also known as CMV, is very common. The majority of adults have antibodies to CMV in their blood (indicating they have been infected) by age 40. Usually, CMV produces no symptoms. However, serious CMV infections can occur in people with impaired immunity (immunocompromised hosts), such as those with AIDS.

People undergoing chemotherapy, which weakens the immune system, or those receiving immunosuppressive drugs for bone marrow or organ transplants, are also vulnerable.

One serious CMV infection is CMV retinitis, which can cause blindness. Retinitis usually begins in one eye, but often progresses to the other eye. Without treatment, progressive retinal destruction will lead to blindness in 4 to 6 months.

Even with continuous treatment, the disease can become worse, causing blindness. This may be because the virus becomes resistant to the drugs, so that the drugs are no longer effective, or because the patient's immune system has deteriorated further.

Patients with CMV retinitis also have a 25-40% of developing retinal detachment, in which the retina detaches from the nerves of the eye, causing blindness. Systemic CMV infection can also occur.

About 25% of people with AIDS develop CMV retinitis. Therefore, AIDS patients with a CD4 count of less than 100 cell/ mm³ should be examined periodically for retinitis, even if they have none of the symptoms listed below.

An HIV positive individual with visual disturbances should seek medical attention and arrange for an exam.