Is addison's disease a rare disease?

Not rare: Addison's disease affects about one in every 20, 000 people. It affects an equal number of men and women, and can affect people of all ages. Addison's disease, also called primary adrenal insufficiency, is much less common than secondary adrenal insufficiency, which is a similar disease that's caused when the pituitary gland fails to send the right stimulating signals (acth) to the adrenal glands.

Yes: Addison's disease is quite rare, thought to affect about 40-60 per million people, or 0.004% to 0.006% of the general population.

Addison's Dz is rare: Accoring to the National Organization for rare diseases- " Approximately 1 in 100,000 people in United States have Addison’s disease. The overall prevalence is estimated to be between 40 and 60 people per million of the general population." REF: https://rarediseases.org/rare-diseases/addisons-disease/

Life threatening: We're lucky it is uncommon, because untreated addison's disease can be fatal. Most addison's in developed countries is autoimmune, and sometimes associated with other autoimmune diseases (eg. Hashimoto's, type 1 diabetes, etc).