A rare disease (sometimes known as an orphan disease) has such a low prevalence in a population that a doctor in a busy general practice would not expect to see more than one case a year. Rare diseases, including those of genetic origin, are life-threatening or chronically debilitating diseases which are of such low prevalence that special combined efforts are needed to address them.
The NIH's Office of Rare Diseases states that, "an orphan or rare disease is generally considered to have a prevalence of fewer than 200,000 affected individuals in the United States" This office was established by H.R. 4013 in 2002. H.R. 4014, signed the same day, refers to the "Rare Diseases Orphan Product Development Act". Similar initiatives have been proposed in Europe.
Rare disease can vary in prevalence between populations, so a disease that is rare in some populations may be common in others. This is especially true of genetic diseases and infectious diseases. An example is cystic fibrosis, a genetic disease: it is rare in most parts of Asia but relatively common in Europe and in populations of European descent. Many infectious diseases are prevalent in a given geographic area but rare everywhere else. Other diseases, such as many rare forms of cancer, have no apparent pattern of distribution but are simply rare.
Eurordis (European Organization for Rare Diseases) estimates that there exist between 5,000 and 8,000 distinct rare diseases, affecting between 6% and 8% of the population.
Rare diseases are usually chronic. Those that are diagnosed are often life-threatening; less severe rare illnesses are seldom properly diagnosed. Eurordis estimates that at least 80% of them have identified genetic origins. Other rare diseases are the result of infections and allergies or due to degenerative and proliferative causes. Symptoms of some rare diseases may appear at birth or in childhood, whereas others only appear once adulthood is reached.