A rare disease (sometimes known as an stray disease) has such a low incidence 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 cause, are life-threatening or chronically devastating diseases which are of such low prevalence that special combined efforts are needed to address them.

As a guide, low occurrence is taken as prevalence of less than 5 per 10,000 in the society.

The NIH's Office of Rare Diseases states that, "an stray or rare disease is generally considered to have a prevalence of fewer than 200,000 influence individuals in the United States"

Rare diseases will differ from population to population, as a disease that is rare in some populations may be common in a further. This is especially true of some inherent and transferable diseases. For example, cystic fibrosis is a rare inherent disease in most parts of Asia but is relatively common in some Eurasian populations. Many infectious diseases, such as humid diseases, are rare outside a given geographic spot.

Eurordis (European Organisation for rare Diseases) estimates that there exist between 5,000 and 8,000 distinct rare diseases, affecting between 6% and 8% of the people.

Rare diseases are usually constant and life-threatening. This is so because, given its rarity, less relentless illness is just not identified as such. Eurordis estimates that at least 80% of them have identified genetic origins. More 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 parenthood is reached.