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.

10. Munchausen's By Proxy

Munchausen's By Proxy is a mental illness by which a person seeks to create the impression that someone in their care (usually their child) is ill.

People with Munchausen's by Proxy inflict injury and unnecessary medical treatments on the person in their care, sometimes resulting in that individual's death.

A famous case of Munchausen's by Proxy is Dee Dee Blanchard, who convinced her daughter Gypsy Rose and several medical professionals and neighbors that the girl was suffering from a litany of medical issues.

Gypsy Rose eventually had her mother murdered to escape her abuse.

9. Lesch-Nyhan Syndrome 

This disorder tends to run in families, and results in an overproduction of uric acid. Resulting symptoms include kidney stones, inflammatory arthritis, and bladder stones.

Treatment for Lesch-Nyan involves the same medication used to treat the gout.

8. Elephantiasis

The Elephant Man's skin condition is transferred to humans by mosquitoes.

Once the infection sets in, the patient's limbs begin to harden.

Their muscle tissues begin to swell up, causing growths to appear on their body. While elephantiasis is treatable, this is only in the early stages.

7. Marie Antoinette Syndrome

There's a story that the night before her execution, Marie Antoinette's hair suddenly turned white.

Sufferers from Marie Antoinette Syndrome may not be facing the guillotine, but they have the same problem.

Luckily, gray hair can be easily fixed with hair dye.

6. Xeroderma Pigmentosum

This genetic disorder makes it difficult to repair damages caused by the sun's UV rays.

For sufferers of Xeroderma Pigmentosum, brief sun exposure can result in severe, painful sunburns.

Other symptoms include dry skin and changes in skin pigmentation.

5. Porphyria

Porphyria results in a build up of porphyrin ( a chemical related to the production of hemoglobin). Symptoms of porphyria include confusion, irritability, blood in urine, and abdominal issues such as vomiting, nausea, and constipation.

King George III of England was a sufferer of porphyria, and the condition is credited with contributing to his mental decline.

Interestingly, sixteenth century and prior Europeans often mistook sufferers of porphyria as suffering from lycanthropy-or being werewolves.

4. Hypertrichosis

Hypertrichosis results in extreme hair growth. This hair growth can take place anywhere on the person's body, occasionally covering the entirety of their face.

Like porphyria above, hypertrichosis is associated with werewolves. Sufferers of the condition often found employment (or exploitation) in nineteenth century circuses.

3. Fragile X Syndrome

Caused by a mutation  of the FMR1 gene, Fragile X Syndrome affects primarily males.

This severe condition results in cognition problems and learning disabilities.

While it cannot be cured, it can be treated through therapy.

2. Cotard's Delusion

This mental illness creates the delusion that the sufferer either does not exist or is already dead. This incredibly isolating illness is further complicated by the issue of misdiagnosis.

Researchers have found that psychiatrists often misdiagnose sufferers of Cotard's Delusion as having depression. And while depression is certainly a symptom of the condition, it is not the same thing and anti depressants alone won't help.

1. Water Allergy

Living things need water to survive—it's the reason we could never move to another planet. We are made up of water, the food we eat contains water.

We cannot survive for more than a week without it. Yet some people have the extreme misfortune of being allergic to water.