OCD may be caused by a combination of factors. It may run in families or may be triggered by life events, eg a new child, changes in relationships or death of a close relative. Particular personality types, eg perfectionists, are prone. OCD has also been linked to increased activity in certain parts of the brain and a decreased level of a natural chemical called serotonin, a chemical that is important in the control of mood. In some children, OCD has developed after a streptococcal infection-a type of bacterial infection.
Attribution: BUPA