Slender man questions I need answered.

Slender: A dare maybe?
Slender Sanatorium: Magic?
Hospice: Trying to change things up a bit>
Elementary: Probably night
Mansion: Extreme paranoia, nausea, blood noses, coughing, amnesia. These are usually the symptoms seen in the vlogs.
Claustrophobia: Probably can't get into the bushes, and even if you could, Slenderman would find you anyway.

Slender; You saw a piece of candy on the other side.

Slender Sanatorium;There IS a door, Slender's just trying to make sure you don't find it. (or it's invisible :p)

Hospice;They're trying to make you feel hopeless.

Elementary;You're stuck in a time freeze, so who knows.

Mansion;Nausea, hearing your heart pounding, riniging in the ears
Claustrophobia;The bushes are actually man-eating monsters, and HOPEFULLY you know a lot better than to go in those things!

I can answer the "Slender Sickness" question. Slender Sickness came from the popular Slenderman ARG Marble Hornets. It consisted of coughing and wheezing. It mostly happened to people who were near him and Proxies. Most other ARG's try to copy this or lack it. In Marble Hornets, the apparent cure for it is strange, rare, unnamed pills in unmarked bottles. Hope this helped you understand Slender Sickness a little more.