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. 

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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).