CMV is a viral family known as the herpesviruses. These viruses have a very neat trick in that they can remain dormant in your body for long periods of time. That is why you could catch the infection but show no symptoms until a later time. They can do this as they make proteins which prevent the immune systems inside your cell from working properly.
What would normally happen is that when a cell notices foreign material within itself, it transports the foreign material to its surface so that immune cells (e.g. white blood cells) can notice and respond accordingly. The virus prevents this process from happening and so continues to lay dormant in cells.
Under some conditions, although I don’t these are yet known, the virus can come out of dormancy and cause its disease, which I think is similar to glandular fever as the virus targets the salivary glands. But again once the visible signs of the disease have passed the virus remains in your cells in a dormant state.