A persistent cough that won't go away could be caused by a number of things but most times a persistent cough is caused by air passages that become inflamed and sensitive or could also be caused by a respiratory tract infection that is serious and known as pneumonia and will need to be treated by a doctor.
If your persistent cough doesn't go away within 18 days a doctor should be seen about it so they can tell whether something more serious is going on or not.
Usually it's nothing serious but it's better to be safe than sorry.