If you had to boil it down, what is cmv?

Cytomegalovirus: CMV is a virus that most people have been exposed to. The virus may cause a mild flu-like illness initially and then becomes latent (inactive). In the immunocompromised, the virus may reactivate and cause inflammation of the colon (colitis), small intestine (enteritis), the esophagus (esophagitis) and the bladder (cystitis).

CMV…: Is one of eight human herpes viruses, characterized by their "hiding out" & reactivating under periods of physical/emotional stress. See also dr. Edwards' answer.