Multiple Personality Disorder
Definition- A rare dissociative disorder in which two or more personalities with distinct memories and behavior patterns apparently exist in one individual.
People affected- From 25,000 to 250,000. - Humanillnesses.com
How to know- If someone has it they may never know because if a different personality comes out, they forget everything that has happened while that personality was out. -Psychcentral.com
Treatments- Psychotherapy, medicine is not recommended for disorders like this.
How to get treatment- Call a therapist and see them and they would probably recommend psychotherapy if anything.
Resources- Wikipedia, Psychcental, Humanillnesses.