What are the world's rarest diseases?

Some of the world's rarest diseases are genetic conditions. In very rare cases, a genetic condition may appear that doctors have never seen before. 
Some of the world's rarest diseases are genetic conditions. In very rare cases, a genetic condition may appear that doctors have never seen before. 

From the primitive cavemen to today's brightest scientists, humans have always faced the many problems diseases pose. Some of these diseases are hereditary conditions passed on from parents to children; others occur as the result of random genetic mutations. Still others are the handiwork of microscopic organisms that have evolved to thrive inside the human body.

Some diseases are regular occurrences throughout human populations, at times swelling to epidemic levels as they spread through large areas. Other diseases occur with far less regularity, though the exact definition of rare disease, or orphan disease, varies from country to country. The National Institute of Health classifies a rare or orphan disease as one with fewer than 200,000 cases in the country. The European Union defines it as any disease that affects five or fewer people out of every 10,000. Whichever definition you use, the answer is the same: Thousands of rare diseases affect millions of people every day.

But what are the world's rarest diseases? It can be a tricky question to answer. Would we necessarily know about them, since they occur so infrequently? New diseases often pop up when another organism's viruses or bacteria adapt to human hosts. Theoretically, new genetic anomalies could also arise. Perhaps the rarest disease is some manmade virus stored away in a government lab.

Given what we know about diseases, how they occur and how they spread, an extremely rare disease would probably have to be one of the following:

  • A genetic disease that rarely occurs
  • A disease mostly eradicated by humans
  • A hereditary condition confined to a particular bloodline

Read the next page to find out about rare diseases like smallpox, Fields condition and a disease that's spread only by eating an infected person's brain (and no, it's not zombiism).

Examples of the Rarest Diseases

David McNew/Getty Images News/Getty Images

Rare diseases are difficult to classify because of their very nature. For our purposes, we've listed examples of the rarest diseases that exist only in laboratories, affect a handful of people or are virtually impossible to contract.


You might not expect one of the world's rarest diseases to be something that has killed millions of people throughout human history. But, if you wanted to catch a case of smallpox, the only sure place to find it would be in a handful of government laboratories.

Smallpox is spread by the variola virus, which causes fever, body aches, and a rash that quickly develops into fluid-filled bumps. These bumps scab and eventually form permanent, pitted scars. The disease spreads chiefly through direct contact with an infected person's skin or body fluids, but can also spread though the air in close, confined environments. The most common form of the disease boasts a 30 percent mortality rate [source: Centers for Disease Control].

However, thanks to the development of a vaccine and the World Health Organization's 1967 eradication campaign, death from smallpox is unheard of today. The last naturally occurring case of the smallpox epidemic was in 1977, and since then only a handful of laboratory accidents have resulted in infections.

Fields Condition

If you have a genetic disease that's really rare, then doctors probably will name it after you. That's exactly what happened to British twins Catherine and Kirstie Fields. The pair were born with a previously undiagnosed neuromuscular disease now known as Fields condition. The disease affects the nerves that cause voluntary muscle movements. As a result, the Fields twins spend much of their time in wheelchairs and have difficulty performing delicate tasks, such as writing.

Scientists are still studying Fields condition, but the twins could conceivably pass the condition to their future offspring. Additional diagnoses may be made as knowledge of the condition grows, and more unrelated cases could develop over time.

While Fields condition may become more widespread, what about rare diseases that show little sign of surviving on their own? One such disease is Kuru.


The average person's chances of contracting or developing Kuru are virtually nonexistent. To catch this fatal neurological disease, you'd have to travel to a remote region in the highlands of New Guinea, find one of the few remaining carriers of the disease, and eat his or her brains.

Kuru belongs to a rare breed of disorders caused by prions. These abnormal proteins induce irregular protein folding in brain cells. This construction technique leads to flawed brain tissue, resulting in progressive and incurable brain damage. The word kuru means "laughing disease," so named because scientists observed fits of hysterical laughing by those affected.

Similar prion conditions, such as Creutzfeldt-Jakob disease (the human variant of mad cow disease) are more common. Kuru only occurs in New Guinea's isolated Fore tribe. The condition first emerged in the 1950s and quickly decimated whole villages. Scientists quickly discovered that the only way to acquire the disease was through the consumption of contaminated brain tissue. The Fore followed cannibalistic funeral rites that required them to cook and eat the dead -- a practice they believed passed on spiritual aspects of the dead to the living. Closely related females and children consumed the brain and contracted the deadly kuru.

When Westerners abolished cannibalism in the region, new incidents of the disease disappeared nearly overnight. Scientists haven't discovered new cases of kuru in any Fore tribesperson born after the late 1950s.

Having a rare disease is half the battle -- you must rely on educated medical professionals to diagnose your condition and help you find treatment. Read on to find out how doctors treat patients with rare diseases.

Treating Rare Diseases

Some countries require pharmaceutical companies to devote time and research to rare-disease treatments.
Some countries require pharmaceutical companies to devote time and research to rare-disease treatments.
iStockphoto.com/Dra Schwartz

Humanity has struggled against disease for thousands of years, and it's a war not likely to end soon. Some of the more devastating epidemics in history either occurred in the last century, such as the Spanish flu, or continue to plague us today. Many of the victories in developing cures, vaccines and treatment programs come only after vast investments of time, money and research. Treating rare diseases is even more difficult.

If the efforts of global organizations and hundreds of scientists are still working to find cures and better treatments for epidemic diseases, such as AIDS, what chance do people with rare diseases have?

It may take time for doctors to accurately diagnose someone with a rare disease. However, once a diagnosis is made, there are a number of organizations that exist solely to help patients with rare diseases find the treatment they need. Additionally, some countries have legislation ensuring that rare diseases receive sufficient attention from researchers and pharmaceutical companies. This way, no matter how rare the disease, patients can expect to find treatment and the necessary orphan drugs, or medications developed to treat rare diseases. The U.S. Food and Drug Administration created the Office of Orphan Products Development to specifically devote resources for these types of diseases [source: FDA].

But does mandated research into rare conditions distract from finding new treatments and cures for widespread diseases? Not at all. The knowledge gained from studying rare diseases often has wide-reaching benefits. For instance, studies of the prion disease Kuru have led to a better understanding of similar, more widespread prion diseases, as well as the threat of mad cow disease.

To pay for the research and development, pharmaceutical companies often place high price tags, which many uninsured individuals cannot afford, on orphan drugs. Additionally, some rare diseases require treatment in a different state or country. This creates further financial hurdles, and many insurance programs place heavy restrictions on such treatment.

Advocacy groups such as the National Organization for Rare Disorders play an important role in ensuring patients have access to the treatments they need for full and productive lives.

To learn more about rare diseases and their advocacy and support groups and organizations, explore the links on the next page.

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More Great Links


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