A Quirk, formerly known as a Meta Ability, is a superhuman ability that a person can possess. It is unique to its wielder, comes in many forms, and is limited to only being able to be acquired through natural means.

The cause of the development of quirks is unknown. However, one researcher did hypothesize that it was caused by the spread of a previously unknown novel disease carried by mice. Others theorized that it was actually a natural evolution of the human race; however, to date, there is no hard evidence to support this theory. What they did know, and informed the public, is that those with Meta Abilities, Quirks, were a subbranch of humanity with new genetic factors.

With the emergence of superhuman abilities, society was broken down by the sudden onslaught of criminals empowered by their newfound 'Superpowers,' and ordinary people became prejudiced against the minority. It was during these dark times that the first groups of 'heroes,' known as vigilantes, were formed. This occurred during the Vigilantes Era, during which time a man named All for One became a symbol of unity in society.

As society tried to adapt to the new status quo, the police forces moved to prioritize leadership and to maintain a balanced status quo. It was decided that quirks would not be used as weapons. To fill that void, the official profession or crime-fighting quirk users emerged. Pro Hereos began to help keep law and order, the first Pro Hero working in Rhode Island of the USA. The rise of a professional type of hero caused the existence of vigilantes to disappear slowly.

With this proliferation of quirks, the government began to regulate their use. A group of quirk users, known as the Meta Liberation Army, attempted to halt this passage of quirk regulations. They believed that the use of Quirks is a basic human right and should be free to use. They rebelled against the governments of several nations for several years before ultimately being defeated. The leader of the Meta Liberation Army was imprisoned, during which time he wrote a book called The Meta Liberation War. It detailed his ideals over liberating quirks, and eventually the story was published. It gained traction many years later.

Currently, roughly eighty percent of the population is made of people who possess a quirk—the number of individuals born without a Quirk continues to decrease with every passing generation. Quirks are thought to be the next stage of evolution in the human race. It is possible to determine if someone has a quirk by analyzing the absence or presence of an extra joint in the person's pinky toe. A person's quirk typically manifests anytime before age four, sometimes even directly after birth. Some late bloomers do not develop their power until a bit later. There are also records of rare individuals who do not develop their work until puberty ages.

A quirk manifestation tends to happen very suddenly, often a surprise to the child. Some people experience pain or side effects as their quirk is attempting to manifest. While quirks are unique from person to person, some powers are more common and widespread than others, such as fire and water manipulation or the power to grow larger body parts. On the other hand, quirks like spatial warping or regenerative healing are very rare.

Not all quirks are singular abilities; some have multiple powers tied into the ability itself. Such is the case with many heteromorphic types who can utilize multiple facets of their quirk's origin; It is even possible for some quirks to take on forms of sentient beings that are connected to the user. The beings are capable of acting and speaking of their own accord but are tied to the quirk holder.

Quirks are an extension of a person's bodily functions. As such, they can be trained and worn out. Similar to how a person works out their muscles, breaking down muscle fibers and building stronger ones, this type of workout applies to quirks as well. If a quirk is overused, the user can suffer a variety of drawbacks.

In most cases, the body of a quirk user has natural adaptions to protect itself from any damage a quirk might do to them. If the user struggles to protect themselves against their quirk, support items are often made to help ease the burden. There are a number of reasons a quirk might harm its user. They might have an ability that is simply too strong for the human body to handle, or they could have a weak constitution in general.

Quirks are also known to have a direct influence on the user's personality. This can sometimes give the user difficulty when trying to fit in with societal norms. In some cases, it can increase the likelihood that the user might develop some more delinquent tendencies. Not only are quirks known to have an impact on personality, but they also have an impact on emotions. This is because quirks are a part of the user's physiology. Some quirks have a more substantial effect on emotions than most, usually quirks that are emotion-based.

Humans are the only species to have developed quirks, but in some incredibly rare cases, it is possible for animals to possess a quirk. There are no concrete facts on how this happens.

Source:
https://myheroacademia.fandom.com/wiki/Quirk