Rare disease are defined as a disease that affect fewer than 200,000 people at any given time. There are about 6800 rare diseases. Most rare disease are connected to mutation of single gene in human body. Check out this link to know the most rare disease in the world.
The most rare disease in the world
1) Hutchinson-Gilford Progeria
This disease was first discovered by Jonathan Hutchinson in 1886 and later described independently by Hastings Gilford in 1897. This disease affects 1 in 8 million children due to genetic mutation. The characteristics are quick aging in childhood with baldness and with large head. Currently there are no cure for this disease.
2) Foreign Accent Syndrome
This is a condition where some people start talking in one or more foreign and unknown language after a stroke or injury to brain which is associated with psychological disorder. The first case was discovered in 1941 and so far only 60 cases have been recorded. The only treatment is extensive speech therapy.
3) Fields Condition
Fields condition or fields disease is so rare that only 2 people ever diagnosed. They are twins Catherine and Kirstie Fields. Its a neuromuscular disease that can cause muscle spasm, restricting their movement and cut their speech ability. So far doctor have no cure for this disease.
4) Aquagenic Urticaria
Aquagenic Urticaria or water allergy is known to have affected only 30 people around the globe, this disease is a very rare painful disease. Those that have this disease cannot be in contact with water as it will cause painful reaction and burning rash. There are no cure for this disease except applying some pain reliever.
Also known as laughing death only occurred to tribal people of New Guinea. This disease starts with maniacal laughter, shaking limb and eventually death within 1 to 3 month. The death was caused by hole in brain. This disease happen due to cannibalism that was exercised by those tribal people where they eat the brain of dead people. Eventually the disease stopped when cannibalism was eliminated.