Parahumans are humans who have undergone a traumatic experience (known as a "trigger event") and awakened superpowers. The slang/colloquial term for parahumans is "cape", which is typically (but not always) used to refer to people who wear costumes, but may refer to Rogues.
Parahumans function like regular human though, unlike regular humans, they have a Corona Pollentia. The Corona Pollentia is the portion of a parahuman's brain that adapts to and allows the parahuman to control their supernatural abilities. The Corona Pollentia seems to awaken during a parahuman's "trigger event". It also seems that parahumans can pass on a similar power to their children without them having to suffer severe mental trauma. It is important to note that while children of parahumans are more likely to gain powers, it is said in the interlude of arc 13 that it is "likely more to do with exposure to parahumans at formative ages than genetics."
Parahumans started to emerge around 1982, if one counts Scion as such, with the first heroes appearing in 1987.
- It is implied that there are a great many capes in the world, but that they still form only a small percentage of the population. Brockton Bay has roughly seventy parahumans that have been introduced and described in the course of the story, implying that capes comprise less than one percent of the population.
- It is also mentioned that there is about 1 parahuman for every 8000 in urban areas.