We all know that as humans we are vulnerable to becoming sick or catching a disease at any moment, especially if we don't take the time to properly look after ourselves.  But if you're healthy right now, or even if you just have the flu or common cold, count yourself among the lucky ones. That's because there are some truly horrific diseases out there.  In this list, we've compiled the 20 of the rarest medical conditions known to man so far.  Brace yourself, these can get a little disturbing.

20. Alexander Disease

This extremely rare neurological disorder was  initially believed to have been developed during childhood.

However, scientists have discovered that Alexander Disease can also be developed later in life.

This degenerative disorder is usually fatal and results in symptoms such as a lowered heart rate, vomiting, and a thin scalp.

19. Sleeping Beauty Syndrome

People suffering from Sleeping Beauty Syndrome have something of an inverse problem to the fairy tale princess.

Where Sleeping Beauty was in a fairy-induced coma for 100 years, people with Sleeping Beauty Syndrome can wake up—they just won't feel fully awake.

Sufferers of this disorder can sleep for up to twenty hours and still experience the confusion and grogginess associated with a lack of sleep.

18. Moebius Syndrome

This neurological disorder affects facial movements.

Caused by poor development of sixth and seventh cranial nerves, patients with Moebius Syndrome often require a feeding tube due to a difficulty in facial movement.

They are unable to smile and move their eyes back and forth.

17. Kuru

While cannibalism is frowned upon in Western cultures, several cultures participate in ritual funerary cannibalism as a way to honor their dead. The brain of a loved one may be consumed in order absorb their wisdom, for example.

Yet eating contaminated human brains can result in Kuru, a disease which attacks the parts of the brain related to coordination and balance.