The transmission of TPF from person to person can be said to at most resemble a disease, and it is on this metaphor that we must draw in order to understand. Like any metaphor, while not being strictly accurate, it may aid the understanding of an otherwise complex topic.
To begin with, the Affliction must be transmissible from person to person in some way. These, as with any disease, are what will be called "vectors", agents that carry and transmit the disease.
The most obvious means of transmitting the Affliction is word of mouth, but it is also among the least effective, the least likely to ensure that the Affliction will find a new host. Compare this to brushing up against someone with a cold.
The next most transmissible means is incidentally-related media, forms of parody or artistic works which, while channelling TPF in one way or another, do not entice in the way that other vectors do. Fictional characters which seem to channel some of the nature of TPF are included in this group. Compare this to being in an elevator while someone coughs.
This is followed by the most widespread vector. Indeed, this particular symptom is rampant. The spread of media related to TPF and his affliction. Records of the Affliction, or works of fiction that depict it mostly accurately, are easily accessible through the internet. Vlogs, Blogs, Fiction, ARGS and artwork depicting TPF all fall under this category. It is here that the lure is almost at its full strength. The way that this information is accessed, it is very easy for someone to dig through and find still more information. Compare this to being in an airplane for a few with someone who has a cold and is coughing and sneezing.
The most infectious vector, indeed the most insidious is that of the Quarantined. This vector manifests itself as a set of behaviours within a Stalked that will most likely lead to a continued infection. The creation of notebooks and other cryptic data, combined with the death or disappearance of the Stalked individual make the draw often irresistible. Compare this with a direct injection of the infectious agent.
The obvious symptom of the disease is the mere apparition of TPF. This is the only true way to confirm an infection. Other common symptoms, the presence of black-outs, the urge to keep a notebook or record information in some way. Repeatedly drawing operator symbols or other representations of TPF, paranoia, the presence of Turned or Agents, drawing or writing that one does not remember making but which has almost certainly be done in their hand. The need to
record everything is not an uncommon one. Symptoms may develop of a second illness 'the Slender-sickness' as well as degeneration of mental stability.
The Affliction is usually a terminal one. The patient either succumbs to the mental component of the Affliction and becomes a Turned carrier, or dies. The only known "treatment" has been to expunge the knowledge of TPF from the mind using amnesiacs. This seems to be largely ineffective as re-infection progresses much more quickly a second time than it did the first. It is, at best, an emergency measure and at worst a grand error in judgment.