This is one disease I wouldn't worry about too much.  It is very rare. 

It is from your adrenal glands not working properly, and not getting enough natural steroids.  It is very treatable.  You just have to take steroids.  These are not the type that athletes take.  They are corticosteroids, and are produced naturally by your adrenal glands in response to stress. 

The person I always think about was John F. Kennedy, who had Addison's.  He lived a pretty normal life with it – if you can call a president's life normal!

I have the opposite, which is called Cushing's Disease.  It is from having to take steroids (prednisone) due to a chronic illness.  There is little danger of getting Cushing's either unless you have to take prednisone for a long time. 

Unless your doctor suggests that you have one of these diseases, I wouldn't worry about it.  Especially Addison's.