The sickness

 The sickness, also known as "slendersickness", is a condition caused by long-term exposure to slenderman and his proxies, usually described as being similar to radiation sickness, as well as a variety of other illnesses. 

Symptoms: 

Stalked: 

Nosebleeds, slight cough, mild amnesia, and possible feever, usually mistaken for a cold 

First encounter:

Nose bleeds, exhaustion, strong cough, vomiting, amnesia, slight signs of radiation poisoning, deja vu, insomnia, the paranoia sometimes starts at this point

More than five encounters:

Victims will start coughing heavily at this point, usually to the point of coughing up blood, vomit with blood, heavy signs of radiation poisoning, severe insomnia, losing large chunks of time, waking up feeling like you've  been running for hours, obsession with slenderman and his symbols, hallucinations, paranoia, agression (these last ones are mostly caused by the insomnia)

These are likely caused by something known as sigma radiation, something supposedly given off by slenderman, as well as possibly being some form of literal virus, being around people who have seen slenderman has the potential to infect people with the sickness. 

(a/n: this shit sucks, been dealing with it since I was 12, I'm trying to find a way to deal with it but... other than avoiding him, there doesn't seem to be any)