The Cycle Of: Obsessive Compulsive Disorder
Obsessive compulsive disorder (OCD) is a mental health disorder characterized by two components: obsessions and compulsions. Obsessions are repetitive and unwanted thoughts, impulses or images that often cause significant distress for the individual experiencing them. Compulsions are any efforts - internal thinking or external actions - that are meant to alleviate the discomfort or fear associated with the obsessions. This description is absolutely not exhaustive, so for more detailed information on OCD I would encourage you to explore the many resources available through the International OCD Foundation.
The cycle of OCD is based on these two core concepts of obsessions/compulsions and the corresponding emotional states in between. When something triggers an obsessive thought (obsession), the thought tends to be persistent and distressing. The individual experiences anxiety associated with the unwanted and distressing thought, often with feelings of fear, discomfort or disgust towards the thought as well. In order to alleviate the anxiety and "let go" of the obsessive thought, the individual feels the urge to address it by engaging in a compulsive behaviour (compulsion). When the compulsion is completed, the individual experiences some temporary relief as the anxiety and thoughts temporarily fade. Unfortunately, due to the pleasant relief that comes from the compulsion, the obsessive thoughts and corresponding compulsions are reinforced within the brain as a good strategy to stay safe from danger. This reinforcement results in that same obsessive thought and corresponding urge to do the compulsion coming back even stronger next time. 
Image Source: https://www.helpguide.org/articles/anxiety/obssessive-compulsive-disorder-ocd.htm
To explain the cycle further, here's a hypothetical scenario. Say a man with OCD has an intrusive thought that he will contract Covid-19 and die. Someone may typically dismiss a thought like this if there's no evidence to support it, but due to his OCD this thought is sticking in his mind and cannot be dismissed. This thought naturally brings about significant anxiety for him, leading him to wipe down all surfaces to limit the possibility of contracting Covid-19 and quell his fears. As soon as he feels he's adequately wiped down all the surfaces, his anxiety is replaced with relief and he is able to move on with his day. Unfortunately, this relief reinforced the ideas that these actions are necessary to stay safe, trapping him into doing these same actions every time the thought of contracting Covid-19 comes into his mind. Over time he may take more and more steps (compulsions) to alleviate the anxiety, until the majority of his day is filled with wiping down surfaces, washing hands, seeking reassurance from others, mentally reviewing all the reasons he won't contract the virus, and researching how the virus is transmitted. Around and around the cycle he goes, slowly increasing the strength of the obsessive thoughts, anxiety and compulsions in the process.
As you can imagine from the above example, the cycle that occurs with OCD can be insidious. It isn't easy to recognize OCD or how it gains strength unless one is looking out for it. Once noticed however, breaking this cycle can be the key to overcoming OCD's grasp.
Breaking this loop takes some tough work. To simply "turn off" our thoughts can be nearly impossible, so research has found the most effective approach is to change how one responds to the anxiety instead. Through a treatment called exposure and response prevention, the individual can practice putting themselves in situations that may trigger the obsessive thought and anxiety, intentionally not do the compulsion or experience the corresponding relief, and thus begin to break the reinforcing cycle. In the process of ERP, the individual breaks the association between the compulsions and feeling safe, learns that obsessions and the corresponding anxiety are false alarms rather than real danger, experiences how anxiety can fade on its own without needing to "do" something about it, and eventually trains the brain to "unstick" from those obsessive thoughts. In the end, the feared thought may or may not arise for the person, but ultimately they will have built up the skills to ignore the feared thoughts without engaging with it or any compulsions.
This intervention is best done with the support of a trained therapist. Having someone who knows how to structure ERP is essential to ensure the individual is set up for success and to safeguard from OCD's tendecy to sneakily introduce new obsessions or compulsions to keep the cycle going. With this treatment and lots of practice, an individual can break the cycle of OCD and regain control over their lives.