Trying to pinpoint what exactly are the rarest disease in the world, or even just the top few is hardly easy. A lot of it comes down to how you categorize things. There are diseases that have not been contracted by humans yet versus those which have. In order to truly be fair, there has to be a certain criteria set in order to distinguish the two types or pages could be filled with diseases that exist only in laboratories since they were engineered rather than naturally occurring. For the purposes of this discussion, all of the diseases listed must be naturally occurring and still a capable of being contracted – even if that possibility is very slim. That is the angle this examination will take. Some of these rare diseases will come as no surprise, and others may very well be shocking.

Fields Condition is widely considered to be perhaps the rarest disease of them all. Fields Condition was named for a pair of British twins, Kirstie and Catherine, who were born with a never before seen neuromuscular disease. The visible signs of the disease are that the afflicted cannot control voluntary muscle movements. This translates into them having extreme difficulty trying perform even very simple tasks like holding a pencil between two fingertips for a period of a few minutes. Very little is known about Fields Condition such as whether the twins can pass it to their offspring or how it even occurred in the first place. Doctors and scientists are on the watch for this, but so far the twins are the only known people dealing with the disease that bears their name.

Kuru is a disease which is considered nearly impossible to contract. The reason for this is that a person would nearly have to intentionally try to become infected with Kuru and go to extraordinary lengths to do so. Kuru can only “naturally” be contracted by being in a very specific remote region of the New Guinea highlands, locating a carrier of the disease, and then eating his or her brain. Not many people do that in their normal daily life. Kuru belongs to the family of disorders caused by prions which causes protein folding in the brain cells. The outward effect is that those carrying the disease go through periods in which they experience uncontrollable hysterical laughing fits. So far no case of Kuru has been reported in any of the Fore Tribesmen born after 1950 when cannibalism was abolished in New Guinea.

Believe it or not a disease which once killed millions, smallpox, is now virtually impossible to contract. At this time it is believed the only way to encounter the disease for sure is in one of the few government laboratories which maintains samples of the disease. The last naturally occurring case of smallpox was reported in 1977, and only a few people working with the disease in laboratories since then have been infected with it. The reason the disease is even in existence at all still is because vaccines are continually being made to safeguard against it just in case it is ever used as biological warfare weapon.

A final possible qualifier for the title of one of the world’s rarest diseases is a PNP (Purine Nucleoside Phosphorylase) deficiency. The lack of PNP leaves the affected unable to fight off almost any virus and requires an urgent bone marrow transplant for any real chance of survival. One of the few people known to have this currently is a two year old Briton who is in need of constant blood transfusions and regular doses of antibiotics to provide even a minimal form of a temporary immune system. Beyond that the attack on his nervous system leaves him almost completely lacking the ability to balance and struggling to even make noise. The life expectancy of a person with PNP deficiency is considered ten years at best.

There you have it, a small sampling of the rarest naturally occurring diseases among humans in the world. Please do note, there are rarer laboratory created diseases which have never infected humans so far as has ever been reported. It is also possible that there is another candidate for this list which like Fields Condition may be undiagnosed only because all other possibilities have yet to be eliminated.