1. A person with whom one is unfamiliar or unacquainted.
2. A foreigner or newcomer in a location: a stranger came to town.
3. An outsider.
4. A person who is unaccustomed to or has not experienced something: they are no stranger to misfortune.
5. A person who is not a member of a group of individuals, as a visitor, or guest: we opened our doors to strangers in need.
All strangers are contained, first and foremost, within a physical body. Although they bear a close resemblance to animals, strangers should not be thought of as organisms in the traditional sense. They are able to move without a muscular system, react to the world without a nervous system, speak without vocal chords, and do not require respiratory, circulatory, or digestive systems to process or regulate energy. As such, strangers can more accurately be described as natural phenomena that take on composite, human-like behaviours.
A stranger's body is relatively simple, and each is composed of only a limited variety of substances and structures. These structures never form complex systems; when a stranger presents with intestines, for example, these organs tend to fill the entirety of its body cavity, with no beginning and no end.
Though no stranger has a digestive system equivalent to that of a typical organism, consumption of matter is observed in some strains. Strangers that "feed" do not draw sustenance by breaking down matter and absorbing its nutrients — rather, as in the case of the slovendri, food disappears inside of them once ingested, leaving behind no by-products. This action appears to sustain those strangers which exhibit these behaviours. Similarly, there are other strains that do not feed at all, or who feed but are not reliant upon the action (or whose health appears linked to temperature, proximity to certain objects, or other variables entirely).
Class denotes a grouping which share common properties or attributes which differentiate or link categories of strangers.
One of the most uniform features among strains is the appearance of eye-like markings. These markings function as sensory organs for many strains, and when these eyes are damaged, it typically blinds the stranger. Some strains seem only able to react to visual stimulus, and do not react to objects obscured by walls or otherwise hidden, while other strains display the ability to track and hone in on targets even when they are out of sight.
A stranger's stability refers to its overall health and constitution. Strangers that are more stable are less prone to illness, more able to thrive in their environment, and tend to display more uniformity in their behaviours, whereas less stable strangers are shorter-lived, more prone to disease and infection, and display more erraticism in their actions. Stability can also be used to estimate the average lifespan of an individual stranger — a stranger with a stability of 35%, for example, has a 35% likelihood of reaching its strain's maximum lifespan.
Stranger infestations appear within urban environments, and require a minimum population density in order to occur. Smaller cities may contain only a few strains isolated in both number and distribution, while bigger cities are host to larger and more diverse populations of strangers, with the most populous cities on earth containing many hundreds, if not thousands of strains. The rate at which an infestation spreads appears to vary based on many unknown external variables. Some cities experience either linear or logarithmic growth, while other cities display rapid propagation of strangers in exponential abundance. Strangers spread the fastest in run-down, economically and socially depressed cities, and infestations spread less quickly (if they appear at all) in highly chaotic environments, such as war-torn cities.
Strangers manifest spontaneously within our world, and do not reproduce sexually. The specifics of this generation vary, with each strain entering the world in a slightly different way. Within an infestation, strains generally appear in numerical order, although not all strains appear in every infestation. The likelihood of a strain's appearance is denoted by its prevalence — thus, a strain with a prevalence of 66% appears in 66% of infestations.
The death of an individual stranger does not affect the strain's spread or propagation in any significant way.
Strangers are not readily visible to people. Despite this lack of visibility, however, a stranger's presence can often continue to be felt, and, even unseen, displays properties that affect human beings.
One in every 100,000 people (or, 0.001% of the total population) is able to perceive and physically interact with strangers. These individuals are known as sensitives. Sensitivity is not linked to any known gene, and it is not carried generationally, although ethnic, religious, and sexual minorities within a population appear more likely to display sensitivity.
Sensitivity appears to exist on a spectrum, and certain individuals are able to see and interact with numerous strains, while others may experience only a handful of interactions spaced out across several decades. Age of onset varies — a select few display sensitivity from birth, while most develop this attribute during adolescence or later.
Encounters between sensitives and strangers occur only in isolated environments, and never in view of non-sensitives. Some strangers are highly predatory in their reaction to sensitives, whereas others display only indifference or curiosity.
Sensitivity varies between individuals. Some are aware of the stranger's unseen presence either through wordless dread or through flashes in their "mind's eye," while others only perceive strangers as ghostly apparitions. There is nothing that any sensitive can do to make a non-sensitive aware of a stranger's physical form.
Although there may be hundreds of thousands of individual strangers within a city, a sensitive is unlikely to observe all strangers present at all times. In this way, an infested city can be compared to a forest — a woods teeming with life but in which a visitor may observe but a single creature, find only empty nests, and hear nothing more than the distant songs of unseen birds.
It is also not only possible, but highly likely for a stranger to be present one day, only to be absent the next, even within enclosed environments. A stranger's constancy refers to the likelihood that an individual stranger will be observable within its environment at any given time — a stranger with a constancy of 15%, for example, can be found 15% of the time, with the moment of its disappearance and re-appearance remaining unobserved.
The presence of animals does nothing to affect a stranger's physical form. An animal's reaction, if any, to a stranger varies depending on species and disposition. Likewise, a stranger's reaction to an animal varies between strains. Some strains react aggressively, while others display complete disinterest.
Many strangers are able to create vocal sounds. To do so, a stranger requires no exhalation, nor do its vocal forms necessitate a specific position of the mouth. Although some strangers do move their mouth as if speaking, the production of audio does not appear contingent on any natural process. These vocalizations range from remarkably human-like intonations to distorted grinding or static sounds.
Linguistic ability varies between strains. Many strains speak their own languages, which vary in complexity from simple bursts and tones to highly structured syntaxes and grammar consistent with human linguistic norms. Semantic significance, however, remains either absent or unobserved.
Some strains mimic existing human languages, with their language typically reflecting the language(s) common to the area in which they appear. Thus, strangers in Tokyo are likely to speak Japanese, while strangers in Montreal are likely to speak either French or English. Although multiple languages can thus be spoken by strangers within a city, multilingualism in individuals is extremely uncommon, with only 0.93% of individuals presenting bilingualism, and 0.008% presenting trilingualism.
Similarly, imitation of syntax ranges from near-perfect, to extremely broken and fragmented. Despite a stranger's superficial imitation of human speech, overall meaning remains nonexistent. Strangers that utilize human languages tend to speak only in abstract statements, repeat words spoken in their vicinity, or ramble in the disordered patterns of an aphasiac.