The concept of disease
The general definition of a disease is as follows: a disease is an impairment of health or a condition of abnormal functioning. WordNet It is a pathological condition of a part, organ, or system of an organism resulting from various causes, such as infection, genetic defect, or environmental stress, and is characterised by an identifiable group of signs or symptoms. Answers.com A patient will define himself or herself as affected by either a disease or a disorder. In this document the word "disease" is used.

The concept of rarity - Figures of rarity
"A rare disease is a disease that occurs infrequently or rarely in the general population". In order to be considered as rare, each specific disease cannot affect more than a limited number of people out of the whole population, defined in Europe as less than 1 in 2,000 citizens (EC Regulation on Orphan Medicinal Products). This figure can also be expressed as 500 rare disease patients out of 1 million citizens. While 1 out of 2,000 seems very few, in a total population of 459 million citizens this could mean as many as 230,000 individuals for each rare disease. It is important to underline that the number of rare disease patients varies considerably from disease to disease, and that most people represented by the statistics in this field suffer from even rarer diseases, affecting only one in 100,000 people or less. Most rare diseases do only affect some thousands, hundreds or even a couple of dozens patients. These "very rare diseases" make patients and their families particularly isolated and vulnerable. It is worth noting that most cancers, including all cancers affecting children, are rare diseases.