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.

16. Alice in Wonderland Syndrome

When Alice arrives in Wonderland, she drinks a potion which makes her shrink.

Sufferers of Alice in Wonderland Syndrome perceive themselves as having the same problem—they believe themselves to be much smaller or larger than they actually are.

This form of body dysmorphia has been linked to episodes of disorientation and distorted perceptions.

15. Morgellons

This controversial condition causes patients to perceive small particles to be crawling out of their skin, often resulting in clawing at their own skin. Patients report sores containing strange fibers.

Yet while doctors concede that sufferers do indeed have sores, these sores are from their own scratching. When tested, the fibers are found to come from clothing, resulting the medical community to come to the consensus that Morgellons isn't a skin condition-it's a delusion.

14. Progeria

This deadly disease results in rapid aging. Affecting one out of every four million people, progeria has a depressing prognosis—it causes heart problems and strokes and is incurable and difficult to treat.

The longest a person with progeria has lived at this time is to age twenty-six.

13. Harlequin-type Ichthyosis

Harlequin-type Ichthyosis is a genetic disorder resulting in thickened skin over the entire body.

Cracks form between the diamond shaped plates of skin, which can potentially lead to infection.

Other issues resulting from the condition include limited movement and misshapen eyes, ears, mouth, and nostrils. There is no cure.

12. Parry Romberg Syndrome

This connective tissue disease results in the tissues on one side of the patient's face degenerating and shrinking.

This causes the appearance of the face caving in. Parry Romberg Syndrome is more prevalent in females, and usually begins between the ages of five and fifteen.

It is also occasionally accompanied by oral and ocular symptoms.

11. Anencephaly

Anencephaly occurs in utero. Occurring when the neural tube fails to close within twenty-three to twenty-six days after conception, it results in the embryo failing to develop a large portion of its brain, skull, and scalp.

Infants who make it to term with the condition rarely survive more than a few hours or days following their birth.