Yes, CMV is very insidious. The majority of the adult population carries antibodies due to exposure at an earlier date. Most don't even know that they have contracted CMV because it mimic the flu or a severe cold, and it is not diagnosed as CMV. It is a relatively harmless virus unless a pregnant woman is exposed. And, as most pregnant women, if after exposure, are asymptomatic, as I was, they don't even know that they have been exposed. Therefore a lot of cases of CMV deafness are listed as "unknown etiology". The only way to confirm congential exposure is to draw a titer at birth to check the baby for antibodies.
My husband and I had a friend who was ill with CMV when I was pregnant. His doctors first thought that he was suffering from flu, but when he fail to recover from the lack of energy within a couple of weeks, they began testing him for Barr Epstein, and made a diagnosis. It was perhaps a month after that when he was properly diagnosed with CMV, and his doctor was at least concerned enough at that point to tell him he needed to let any pregnant women he had been around to know that they may have been exposed. I immedicately informed by OB-Gyn who drew a titer on me the same day, even though I had no symptoms, nor did I ever experience symptoms. It was confirmed at that time that my titer was elevated, and my fetus had been exposed in utero.