Rare diseases are also known as “orphan” diseases due to the lack of ‘desirability’ for clinical and experimental researchers. There is no single definition for rare diseases. Some definitions depend on the number of people affected by the disease, whereas other definitions depend on the severity of the disease or the availability of treatments. Medical literature and national health plans define rare diseases as those affecting people from a range of 1 in 1000 to 1 in 200,000.
According to the National Institute of Health (NIH), there are about 7000 rare diseases. One of these diseases is present in 30 million people, or 1 in 10 in the United States. The symptoms of some rare diseases may appear during birth or at childhood, whereas others appear at adulthood. Rare diseases that are caused by alterations in the gene are known as genetic diseases. Most rare diseases are genetic and others occur as a result of infections and allergies or abnormalities in proliferation and degeneration. About 30% of children suffering from rare disorders die within the age of five.
The prevalence (number of people living with a disease at a particular moment) of rare diseases can vary among different populations, which means a disease that is rare in one population can be common in another. This is shown to be true especially for infectious diseases and genetic diseases. One example of such a genetic disease is cystic fibrosis, which is rare in most regions of Asia but common in Europe.