Obsessive-compulsive disorder, or OCD, is a medical disorder that causes repetitive, unpleasant thoughts (obsessions) or behaviors (compulsions) that are difficult to control. Unlike ordinary worries or habits, these obsessions and compulsions may consume significant amounts of time (more than an hour per day), may interfere with a person's daily schedule, and may cause significant distress. OCD affects approximately one percent of children and adolescents. The tendency to develop this disorder involves complex genetic and environmental factors.

(Massachusetts General Hospital)

People with OCD may have symptoms of obsessions, compulsions, or both. These symptoms can interfere with all aspects of life, such as work, school, and personal relationships.

Obsessions are repeated thoughts, urges, or mental images that cause anxiety. Common symptoms include:

• Fear of germs or contamination
• Unwanted forbidden or taboo thoughts involving sex, religion, and harm
• Aggressive thoughts towards others or self
• Having things symmetrical or in a perfect order

Compulsions are repetitive behaviors that a person with OCD feels the urge to do in response to an obsessive thought. Common compulsions include: Excessive cleaning and/or hand washing Ordering and arranging things in a particular, precise way Repeatedly checking on things, such as repeatedly checking to see if the door is locked or that the oven is off Compulsive counting Not all rituals or habits are compulsions. Everyone double checks things sometimes. But a person with OCD generally:

• Can't control his or her thoughts or behaviors, even when those thoughts or behaviors are recognized as excessive
• Spends at least 1 hour a day on these thoughts or behaviors
• Doesn’t get pleasure when performing the behaviors or rituals, but may feel brief relief from the anxiety the thoughts cause
• Experiences significant problems in their daily life due to these thoughts or behaviors

Some individuals with OCD also have a tic disorder. Motor tics are sudden, brief, repetitive movements, such as eye blinking and other eye movements, facial grimacing, shoulder shrugging, and head or shoulder jerking. Common vocal tics include repetitive throat-clearing, sniffing, or grunting sounds.

(National Institute of Mental Health)

The cause of obsessive-compulsive disorder isn't fully understood. Main theories include:

• Biology - OCD may be a result of changes in your body's own natural chemistry or brain functions. OCD may also have a genetic component, but specific genes have yet to be identified.
• Environment - Some environmental factors such as infections are suggested as a trigger for OCD, but more research is needed to be sure. 

(Mayo Clinic)

Obsessive compulsive disorder (OCD) can be treated. The treatment recommended will depend on how much it's affecting your life.

The 2 main treatments are:

• Talking therapy – usually a type of therapy that helps you face your fears and obsessive thoughts without "putting them right" with compulsion.
• Medicine – usually a type of antidepressant medicine that can help by altering the balance of chemicals in your brain.

A short course of therapy is usually recommended for relatively mild OCD. If you have more severe OCD, you may need a longer course of combined therapy and medicine.

(National Health Service - UK)