About one in ten people around the world suffer from phobias, and there are hundreds of different types.
Phobias are a type of anxiety disorder where you have a persistent, irrational or excessive fear of certain things, places or situations, which impacts upon your life. You're probably familiar with the common ones, such as fear of heights, spiders, or confined spaces.
But then there are phobias which are a little more unusual. Just this week the Swedish traffic authority downsized a man's fine for his traffic offences after learning about his 'window envelope phobia', or fear of official mail.
So here are some other phobias you may not have heard of before.
This is a rare social phobia which is characterised by an overwhelming and irrational fear of other people's opinions, making dinner party arguments or work meetings hellish experiences.
It is thought to come from previous encounters where the person has not been able to properly express their opinion and can reflect a fear of confrontation. One example would be emotional abuse stemming from constant criticism.
Summer is not a good time of year for you if you suffer from genuphobia, or the fear of knees.
It's relatively rare, and varies in severity. Some people are afraid of seeing uncovered knees in person, while others are afraid of bare knees on TV or film. People may be afraid of all knees, or only their own, and some people are scared of kneeling. If you have a traumatic knee injury, you may be more likely to develop genuphobia.
English woman Stephanie Cockerill first experienced genuphobia when she was just 15, and had a violent reaction to her boyfriend's knees brushing against hers in bed.
Whenever she sees a knee she feels sick, starts shaking, and sometimes even gets a panic attack.
"I hate the way knees feel and my worst nightmare would be if someone else's knee touched mine," Cockerill told the Mirror. "I never cross my legs like other women because I can't stand the feeling of the back and front of my knees touching each other. If I could have an operation to remove my knees I would."
The severe anxiety of being without a mobile phone or beyond mobile contact is one that many people can relate to. In fact, psychologists say smartphone addiction is on the rise, and the addicts are getting younger.
A recent survey of 1000 South Korean students found that 25 per cent were considered to be addicted to smartphones.
A 19 year old South Korean student Emma Yoon (nit her real name) has been undergoing treatment for nomophobia since April 2013.
"My phone became my world. It became an extension of me," she told the BBC.
"My heart would race and my palms grew sweaty if I thought I lost my phone. So I never went anywhere without it."
The fear of cotton wool balls is a sensory phobia which is believed to affect a handful of people around the world. In fact, Michael Jackson is believed to have suffered from sidonglobophobia.
Experts generally believe this fear develops in childhood in response to a negative or traumatic experience somehow connected to cotton balls.
Michelle Andrews has been afraid of cotton wool since she was 6 years old, and even the thought of it sends her into a cold sweat.
"Wanting to wrap someone up in cotton wool is supposed to be a way of showing that you care," she told the BBC. "In my case, it would be the ultimate form of torture."
This fear of holes is a condition whereby sufferers have an emotional reaction when viewing clusters of objects, usually holes.
The images can be natural objects like honeycomb or a lotus seed head, or man-made items like aerated chocolate or stacked industrial pipes. Up to 15 per cent of people become upset after looking at images of clustered holes or bumps, according to research on trypophobia.
A 2013 study in the journal Psychological Science quotes one sufferer who describes how she feels when she looks at an image of holes: ""[I] can't really face small, irregularly or asymmetrically placed holes, they make me like, throw up in my mouth, cry a little bit, and shake all over, deeply."
Heliophobia, or the fear of sunlight, has been called the 'vampire' phobia.
Sufferers experience panic attacks when they go out in the sunlight, and prefer to stay inside out of the sun's rays.
For some it's a type of health anxiety, fostered by a belief that minimising their exposure to the sun will minimise their chances of developing skin cancer, while those with body image issues may be concerned by the premature aging effects of sun damage.
For heliophobia sufferer Lucy Jefferies the fear was sparked by a cancer scare when she had a lump in her neck removed.
"When I feel the sun on my skin it doesn’t feel warm, it feels like it’s sort of penetrating my skin with cancer," Jeffries told The Sun. “I’ve got to a point where I just don’t go out any more. Instead I get my shopping online and stay in.”
A fear of chocolate might just sound like your worst nightmare, but sufferers of Xocolatophobia can find it unbearable to look at, let alone eat, the stuff.
Christmas and Easter are the worst times of year for Andrew Bullock, who can trace his chocolate phobia back to his childhood when his mother also suffered from Xocolatophobia.
"If I touch chocolate I feel dirty. Even if I wash my hands I feel like it's still there," he told The Express. "If I see someone else eating it I start to feel worried about the chocolate getting on me.
"When I go out for dinner and someone has a big chocolate thing for dessert it freaks me out and I feel stressed."