1) Why February 28?
Actually, Rare Disease Day takes place on February 29, in leap years—the rarest day of the calendar is a fitting choice to highlight the rarity of these diseases. In non-leap years, such as 2025, it occurs on February 28 instead.

2) Who picked this date?
Rare Disease Day was established by EURORDIS, the European Rare Disease Organization, and first observed on February 29, 2008. Since then, over 100 countries and regions have followed suit, for example the USA in 2009, South Korea in 2024.

3) How rare is a rare disease?
This is a bit confusing, because different areas have slightly different criteria. A rare disease affects fewer than 

• 5 in 10,000 in the EU and UK, 
• 200,000 in the USA (around 5.5 in 10,000)
• 50,000 in Japan (around 4 in 10,000). 

The World Health Organization (WHO) defines a disease as rare if it affects fewer than 6.5 in 10,000 people.

4) How many rare diseases are out there?

Estimates vary wildly, ranging from 5,000 to 8,000, depending on the criteria used to define a rare disease. A 2019 attempt to harmonize these definitions concluded that there are more than 10,000 rare diseases [1].

5) How common are rare diseases collectively?
A 2020 study estimated the global prevalence of rare diseases at 3.5–5.9%, which equated to 263–446 million people worldwide in 2017 [2]. As this study included only 6,000 rare diseases, and estimates suggest there are way more (1), it is likely that the number of people with a rare disease is even greater. 

6) Which is the rarest disease?

This may be Fields condition, a neuromuscular disorder named after the two only patients [3]: Welsh identical twins Catherine and Kirstie Fields. The disorder affects the sisters’ nerves and causes their muscles to progressively waste away and painfully twitch. The cause of the disease is currently unknown.

7) Are all rare diseases caused by faulty genetics?

No, “only” 70% to 80% of rare diseases have a genetic cause [2]. Non-genetic causes include 

• infections (bacteria, viruses, fungi, prions…),
• toxins (such as asbestos, which causes a rare lung cancer), 
• nutritional deficiencies (a diet lacking a vitamin or mineral),
• injuries and,
• adverse reactions to drugs and foods.
  

8) How often do rare diseases affect children?

Approximately 75% of rare diseases affect children, 30% of whom die before their 5th birthday [4].

9) Is “orphan disease” a synonym for “rare disease”?
Not exactly. An orphan disease is rare and it also lacks funding because investors don’t think the sales will allow them to recoup the costs of research and development.

10) Where should I look for more information?
Some of the most authoritative are 

• National Organization for Rare Diseases (NORD) database: lay-friendly information on rare diseases,
• Orphanet: database on rare disease and their diagnosis, care, treatment,
• Genetic and Rare Diseases (GARD) information center: on rare diseases, genetic and not-genetic ones,
• Online Mendelian Inheritance in Man catalog (OMIM): on common and rare genetic traits and disorders.