1. Rare diseases are not classified like other diseases
Disease classifications generally look at similarities in causes or symptoms. Rare diseases are different, as the classification is based on prevalence. In Europe, a rare disease is defined as one that affects fewer than 1 in 2,000 people, while the American definition states that a rare disease is one that affects fewer than 200,000 people in the USA [1,2].
2. Rare diseases show heterogeneity in many aspects, including origins, population affected, and signs and symptoms
Rare diseases can be inherited (e.g. mucopolysaccharidoses ), or caused by bacterial or viral infections (e.g. anthrax  and rabies ), or environmental factors (e.g. malignant peritoneal mesothelioma caused by asbestos exposure ). People of any age can be affected by a rare disease, although it’s estimated that 50–75% affect children [7,8]. The signs and symptoms of rare diseases are also highly variable and complex – even patients with the same disease can experience different symptoms, ranging from mild to severe, with slow or rapid progression.
3. Some rare diseases affect only one individual in the world
The prevalence of rare diseases varies widely. For some rare diseases only one case has been reported (e.g. some congenital disorders of glycosylation), while others can affect hundreds of individuals (e.g. tens or hundreds of cases of some immunodeficiency syndromes are known), or even hundreds of families (e.g. some types of epilepsy are reported to affect about 100 families ).
4. About 7,000 types of rare disease are currently known to exist 
Although each individual disease may only affect a few people, the large number of different types of rare disease means that an estimated 350 million individuals are affected globally . Moreover, approximately 5 new rare diseases are identified each week, so we can expect the number of affected individuals to grow .
5. On average, it takes nearly 5 years for a rare disease to be correctly diagnosed 
Diagnosis of rare diseases is challenging. The rarity of some conditions may mean that a particular doctor has never encountered the rare disease their patient is affected by. Furthermore, many rare diseases include common signs and symptoms, leading to misdiagnosis, which delays appropriate disease management. Misdiagnoses and the resulting treatment delays can be enormously detrimental to the patient, and symptoms developed before diagnosis may have an irreversible impact on the patient.
The overall prevalence and heterogeneity of rare diseases result in a large number of patients affected by these complex, life-changing disorders. The impact of rare diseases on the day-to-day life of patients and their families is compounded by diagnostic delays and a challenging environment for the development of therapies that target rare diseases. Limited knowledge of rare diseases may reduce the likelihood of patients initiating treatment in a timely manner, while the high costs of running clinical trials and manufacturing drugs for a relatively small number of patients means that, when available, treatments are frequently expensive.
The Rare Disease Day campaign aims to combat these issues by raising awareness among the public, healthcare professionals, researchers, industry, and policy decision–makers. This global campaign aims to bring down barriers to timely diagnosis, and improve access to available treatments and comprehensive care. By supporting coordinated, international research and clinical trials, it is hoped that new therapies will be developed for the many rare disease patients who rely on palliative strategies. Starting as a European initiative in 2008, the campaign is now global, with 94 countries holding events in 2017. There is still much work to be carried out to ensure rare disease patients are receiving optimal treatment and support, and today organisations across the globe will be supporting the Rare Disease Day campaign to ensure this goal is reached.
To find out more, please visit http://www.rarediseaseday.org/.
1. Orphanet. About rare diseases. Available at: http://www.orpha.net/consor/cgi-bin/Education_AboutRareDiseases.php?lng=EN. Accessed February 2018.
2. National Organization for Rare Disorders. Resources & FAQs: What makes a disease rare? Available at: https://rarediseases.org/for-patients-and-families/information-resources/resources-faqs/. Accessed February 2018.
3. Lehman TJA, Miller N, Norquist B et al. Diagnosis of the mucopolysaccharidoses. Rheumatology 2011; 50 (Suppl 5): v41–v48.
4. Genetic and Rare Diseases Information Center. Bacterial infections. Available at: https://rarediseases.info.nih.gov/diseases/diseases-by-category/2/bacterial-infections. Accessed February 2018.
5. Genetic and Rare Diseases Information Center. Viral infections. Available at: https://rarediseases.info.nih.gov/diseases/diseases-by-category/25/viral-infections. Accessed February 2018.
6. Asbestos.com. Peritoneal mesothelioma. Available at: https://www.asbestos.com/mesothelioma/peritoneal.php. Accessed February 2018.
7. Rare Disease UK. What is a rare disease? Available at: http://www.raredisease.org.uk/what-is-a-rare-disease/. Accessed February 2018.
8.RareDiseaseDay.org. What is a rare disease? Available at: http://www.rarediseaseday.org/article/what-is-a-rare-disease. Accessed February 2018.
9. Orphanet Report Series. Prevalence and incidence of rare diseases: Bibliographic data; November 2016. Available at: http://www.orpha.net/orphacom/cahiers/docs/GB/Prevalence_of_rare_diseases_by_alphabetical_list.pdf. Accessed February 2018.
10. Global Genes. Rare diseases: Facts and statistics. Available at: https://globalgenes.org/rare-diseases-facts-statistics/. Accessed February 2018.
11. Engel PA, Bagal S, Broback M et al. Physician and patient perceptions regarding physician training in rare diseases: The need for stronger educational initiatives for physicians. J Rare Disord 2013; 1 (2). Available at: http://www.journalofraredisorders.com/pub/IssuePDFs/Engel.pdf. Accessed February 2018.