Obsessive Compulsive Disorder
OCD is a term who's definition has been much less serious. Everyone does things like press the lock button on your car twice just to make sure, which is normal. There are however people who have such a compulsion to wash their hands to avoid germs that they become cracked and bleed, and even then continue to wash them. OCD is characterized by the need to check things over and over again, or preform certain routine rituals or routines.
OCD can run in families, but no one is quite sure why some people have it and some don't. Environments may also play a role in the development of OCD along with stress but more research is needed to confirm this.
Treatment for OCD can vary from psychotherapy to forms of medication. Psychotherapy is especially useful for patients with OCD because it can teach them different ways of thinking or reacting to situations to reduce the amount of stress or anxiety they undergo. Medications such as anti-depressants and anti-anxiety drugs are often used.
Dissociative Personality Disorder
DID, formerly known as Multiple Personality Disorder, is a disease in which a person has two or more defined states or identities. These states can be completely different from the original identity even having different handwriting. Patients with DID often report frequent memory loss or "lost time", out of body experiences, and the inability to explain large parts of their childhood.
DID is believed to be caused by prolonged childhood trauma like sexual, emotional, or physical abuse. The lack of nutrition or shelter can also contribute to the development of DID.
The most common form of treatment for DID is psychotherapy. The goal of the therapy is to unite the different personalities into one whole identity, this often involves attempting to work through the trauma's that initially caused the dissociation. DID is not an organic disorder so there is no medication to treat it, but some anti depressants are prescribed to prevent suicidal thoughts or actions.