Yes, atrium enlargement is the result of a disease.
There can be a disruption in normal blood flow is one of the most common causes of left atrial enlargement. Also, the underlying cause can be obesity. The cause can be related to stress, high blood pressure, narrow mitral valve (this is the valve that opens to pass blood to the lower chamber...a narrow valve creates more pressure inside the atrium and the chamber will enlarge.
The left atrium is vulnerable to any infection from lungs.
Hope this helps and if you have any further questions you are welcome to respond. Take care.