Definitions of common and rare disease Is there an epidimiological way to define **rare** and **common** diseases ?

While "common disease" has obscure incidence and prevalence definitions, "Rare" or "orphan disease" is more defined.

In the US, the rare disease defined as a condition which affect one (or less) of 200,000 people in population. However, this definition can be different in other countries, p.e. in Japan 1:50,000 or less.

We can assume that "common" disease is much frequent than "rare" one, thus the incidence and prevalence are much greater than per "rare disease" definition. However, again, there is no agreement of clear cut of "common disease" incidence and prevalence.