While many of us have unwanted thoughts from time to time, we often can push them aside and move on.
The same cannot be said for people with OCD; for them, unwanted negative thoughts can take control of their lives.
OCD (obsessive compulsive disorder) is an anxiety disorder that affects more than 500,000 people in Australia.
Dr Emily O'Leary, director of the OCD Clinic in Brisbane, said the disorder could manifest in many different forms.
"OCD is categorised by intrusive thoughts, images and impulses that cause such anxiety that they feel like they need to do repetitive actions to make the anxiety go away," she said.
"An example of that is a person feeling dirty or contaminated and they begin to engage in repetitive handwashing to make themselves feel better.
"I often use the contamination one as it's what people recognise the most."
But Dr O'Leary said that at her clinic the most common form of OCD was harming or aggressive obsessions.
"They get intrusive thoughts about bad things happening to people they love," she told ABC Radio Brisbane's Emma Griffiths.
"It can be life throwing bad things at them or inappropriate sexual thoughts.
"What they then do is a number of compulsive behaviours to keep them safe."
What causes OCD?
Dr O'Leary said genetics often was a contributing factor.
"It's a combination of factors that work together in a perfect storm to cause OCD," she said.
"Factors such as personality, temperament, early upbringing and if it runs in the family."
She said it was not possible to have OCD without knowing.
"There is a real difference between being quirky and obsessive and having OCD.
"People with OCD, even if they haven't sought treatment, know it's an issue as it interferes in their day.
"They spend vast amounts of times engaging in these rituals and it causes stress."
Living with OCD
Dr O'Leary said there was a big gap between diagnosis and people seeking treatment, often more than 10 years.
"The main reason that they don't seek help is that they are embarrassed and ashamed about the symptoms," she said.
"We aim to reduce the anxiety and the OCD and form a realistic outcome through a treatment plan.
"We help people accept that the thoughts can be in their head but they don't have to do anything with them."
If you or someone you know are experiencing symptoms of OCD talk to your GP or contact Lifeline on 13 11 14.