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.