Slender Sickness or The Sickness is apparently the result of being stalked by or even having a single encounter with The Slenderman. There are a range of symptoms which intensify the more aggressively he stalks you. The sickness can bee seen in the popular YouTube series Marble Hornets Series. Some theories claim slender sickness is directly linked to radiation poisoning and that this is one of The Slenderman's powers.
The Symptoms usually start out with just slight illness and nose bleeds. Later resulting in strong cold or flu like symptoms, including exhaustion, nausea, vomiting and signs of minor radiation poisoning. At the latest and most severe stages of this illness you are likely to show symptoms like coughing and vomiting blood, violent convulsions obvious signs of radiation poisoning and even amnesia.
There are many explanations as to why having an encounter with The Slenderman results in such illnesses. This is a list and brief description of the more popular theories.
Radiation is probably the most common theory for The Sickness. There are two popular radiation theories, electromagnetic radiation and sigma radiation.
It is believed that The Slenderman has the ability to intentionally emit sound waves that create electromagnetic radiation to disrupt human technology. Exposure to this kind of radiation in low amounts can result in infections, bleeding and anemia, higher levels of exposure can result in abdominal pain, nausea, vomiting and decreased level of consciousness. Untreated high levels of exposure are usually fatal.
The idea that The Slenderman is attempting to disrupt human electronics is consistent with most videos recording and games like Slender: The Eight Pages and in all of the Slenderman's Shadow series where looking at The Slenderman causes the video to become blurred and distorted with static and cut out soon after seeing him.
The sigma radiation theory was first mentioned on the YouTube series EverymanHYBRID in the research notes of a physics student. He discovered the radiation on a piece of fabric believed to be part of The Slenderman's suit, which is unlike any other radiation known to man. Later in the series the entire main cast discover that their blood tests positive to the presence of sigma radiation. It is unknown what the presence of this sigma radiation in humans means.
Infrasound or low frequency sounds have been proven to effect feelings, sight and the physical world to human beings, explaining some paranormal experiences. Because the frequency is so low, the sound is inaudible. When exposed to frequencies between 0Hz and 100Hz, at up to 150-155dB the individual may experience headaches, changes in respiratory rhythm, post-exposure fatigue, gagging sensations and coughing. Frequencies around 19Hz can cause vibration of the eyeballs, distorting vision and under the right conditions cause visual hallucinations.