What is a “rare” disease?

A disease is defined as "rare", when it affects 1 in 2000 individuals[1]

Rare diseases - The facts


million people living with a rare disease worldwide

3.5% – 5.9%

of the world’s population


of rare diseases are genetic

More about rare diseases

Over 6,000 different rare diseases have been identified to date.

Seventy-two percent of rare diseases are genetic while others are the result of bacterial or viral infections, allergies and environmental causes.

Fifty percent of the genetic rare diseases affect children. According to data of the World Health Organization (WHO), approximately 400000 people (5% of the Bulgarian population) are affected by them, which means that 1 out of 20 people has a rare disease.[2]

Rare diseases are life-threatening or chronically debilitating diseases with a low prevalence and a high level of complexity.

The concept of “rareness” involves a great degree of relativity. Although each rare disease affects a very small part of the population of a country, taken together they pose a serious problem to any public health system due to the large number of nosological entities.

The impact

Rare diseases are characterized by a wide diversity of symptoms and signs that vary not only from disease to disease but also from patient to patient suffering from the same disease.

Due to the low prevalence of each disease, medical expertise is rare, knowledge is scarce, care offerings inadequate and research limited. Despite their great overall number, rare disease patients are the orphans of health systems, often denied diagnosis, treatment, and the benefits of research.

Relatively common symptoms can hide underlying rare diseases leading to misdiagnosis and delaying treatment. Typically disabling, the quality of life of a person living with a rare disease is affected by the lack or loss of autonomy due to the chronic, progressive, degenerative, and frequently life-threatening aspects of the disease.

The fact that there are often no existing effective cures adds to the high level of pain and suffering endured by patients and their families.

How can the Rare Disease Day and the 20th Hero campaign make a difference?

The Rare Disease Day raises awareness for people around the world living with a rare disease and their families and carers.

The long-term cause of the Rare Disease Day campaign is to help procure equitable access to diagnosis, treatment, medical and social care, and social opportunity for people affected by a rare disease.

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