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Rare Diseases A-Z: Orphan Diseases Defined

Author: Ian C. Langtree - Writer/Editor for Disabled World (DW)
Published: 2020/06/09 - Updated: 2025/12/07
Publication Type: Informative
Category Topic: Informational Lists - Related Publications

Page Content: Synopsis - Introduction - Main - Insights, Updates

Synopsis: This reference provides an extensive alphabetized catalog of rare diseases and establishes critical definitions distinguishing rare conditions across regulatory jurisdictions. The guide addresses the fact that approximately 7,000 rare diseases exist globally and over 300 million people live with such conditions - meaning an average of one in seventeen people will encounter a rare disease during their lifetime. The resource proves invaluable for patients, caregivers, and healthcare practitioners seeking clarity, as it explains how different regions define rarity: the United States identifies rare diseases as affecting fewer than 200,000 people, while the European Union uses a prevalence threshold of less than one in 2,000. For individuals navigating diagnosis and treatment decisions, seniors managing complex health conditions, and disabled people searching for community resources and medical context, this comprehensive breakdown offers both a functional reference and the authoritative foundation necessary for informed healthcare conversations - Disabled World (DW).

Introduction

Defining Rare Diseases

The term orphan disease is sometimes used as a synonym for rare disease. Most genetic disorders are rare diseases, but not all rare diseases are caused by genetic factors. There are approximately 7,000 rare diseases (RD). Rare diseases include, some very rare infectious diseases, rare forms of autoimmune disorders, and rare cancers. Global Genes estimates over 300 million people worldwide are living with a rare disease. On average 1 in 17 people will be affected by a rare disease at some point during their lives.

Main Content

There is no single, widely accepted definition for rare diseases. Some definitions are based on the number of people living with a disease, and other definitions include factors, such as existence of adequate treatment(s) or the severity of the disease. Rare disease definitions used in medical literature and by national health plans are divided, with definitions ranging from 1 in 1,000 people to 1 in 200,000 people.

Rare Disease Awareness Day is held globally on February 29 - February 28 on non-leap years. The awareness day was originally started by the European Organization for Rare Diseases and is now recognized globally. The rare disease awareness ribbon is a zebra-striped ribbon.

Incomplete List of Rare Diseases

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

A

B

C

D

E

F

G

H

I

J

K

L

M

N

O

P

Q

R

S

T

U

V

W

X

Y

Z

Insights, Analysis, and Developments

Editorial Note: The persistence of rare disease remains one of modern medicine's paradoxes - these conditions are individually uncommon, yet collectively affect a staggering share of humanity. What makes a disease "rare" is not a fixed biological property but rather a human-constructed category that shifts depending on geography, policy, and pharmaceutical economics. The original impetus for these definitions came from practical necessity: the United States Orphan Drug Act of 1983 recognized that drug manufacturers had no profit incentive to develop treatments for small patient populations, creating a legislative framework to encourage therapeutic innovation where market forces alone would fail. Today, as this resource demonstrates, that definitional infrastructure matters profoundly to patients waiting for diagnosis, researchers seeking funding, and policymakers allocating healthcare resources. Understanding that rarity is relative - and often culturally determined - reminds us that the absence of an easy medical label should never mean the absence of medical attention - Disabled World (DW).

Ian C. Langtree Author Credentials: Ian is the founder and Editor-in-Chief of Disabled World, a leading resource for news and information on disability issues. With a global perspective shaped by years of travel and lived experience, Ian is a committed proponent of the Social Model of Disability-a transformative framework developed by disabled activists in the 1970s that emphasizes dismantling societal barriers rather than focusing solely on individual impairments. His work reflects a deep commitment to disability rights, accessibility, and social inclusion. To learn more about Ian's background, expertise, and accomplishments, visit his .

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APA: Disabled World. (2020, June 9 - Last revised: 2025, December 7). Rare Diseases A-Z: Orphan Diseases Defined. Disabled World (DW). Retrieved January 30, 2026 from www.disabled-world.com/definitions/lists/rare-disease.php
MLA: Disabled World. "Rare Diseases A-Z: Orphan Diseases Defined." Disabled World (DW), 9 Jun. 2020, revised 7 Dec. 2025. Web. 30 Jan. 2026. <www.disabled-world.com/definitions/lists/rare-disease.php>.
Chicago: Disabled World. "Rare Diseases A-Z: Orphan Diseases Defined." Disabled World (DW). Last modified December 7, 2025. www.disabled-world.com/definitions/lists/rare-disease.php.

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