Cytomegalovirus and the Epstein Barr virus both cause mononucleosis, according to papers published on PubMed. They are also associated with immune diseases, such as rheumatoid arthritis, polyarteritis nodosa and multiple sclerosis, and anyone with these diseases is likely to have one of the two viruses.
Cytomegalovirus is a very common virus and is a member of the herpes virus family, states MedicineNet. It is spread through direct contact with bodily fluids. Though it is harmless in a healthy person, it can cause dangerous illness in a person whose immune system is weakened. It can cause blindness, pneumonia or encephalitis, and infected mothers can pass a CMV infection on to their fetuses or through breastfeeding.
The Epstein Barr virus, or human herpesvirus 4, is a cousin of CMV, reports the Centers for Disease Control and Prevention. It is also a very common virus and is also spread through bodily fluids. Like CMV, it is especially dangerous for people who have weak immune systems. The Epstein Barr virus causes a type of mononucleosis called the "kissing disease," as it is spread through saliva, states Mayo Clinic. It can also be spread through sneezing, coughing or sharing eating utensils, though it's not as easy to catch as the common cold.