Generally speaking, there's nothing wrong with fear. It's a great tool in our emotional arsenal that helps us appropriately deal with danger or threat. Our proverbial ancestors would have ended up dead rather quickly if fear hadn't told them not to pat every sabre-toothed tiger they met.

But as with any survival mechanism, human emotions can sometimes go out of whack, which is why some us also have to deal with a type of anxiety disorder known as phobia. To be clear though, you're not phobic if you simply feel a bit iffy around spiders. "To qualify as a phobia, a fear must be lasting, intense, and seen by the sufferer as excessive and irrational," psychologist Nick Haslam writes at The Conversation. "It must also be a source of distress or impairment in the person's occupational life and social relationships." These distressing experiences often come with a range of physical symptoms, such as panic, sweating, nausea, and shaking.

Phobias affect roughly one in 10 people worldwide, and you certainly know all the most common ones - spiders, snakes, heights, and blood. All of these can be easily explained by looking back to our ancestors and noting some of the immediate threats in their environment, hence the relative frequency of these types of excessive fears.

But we also manage to develop phobias based on traumatic experiences, other people's fears, or even just from learning terrifying information about the trigger - regardless of whether there's any truth to the facts. It could well be that some people develop a fear of clowns because they were frightened by one when young (and hey, who can blame them).

Even though the scientific literature is sparse when it comes to accounts of rare phobias, there is no shortage of anecdotes, and lists of phobia names run in the hundreds. Here are some of the especially rare ones we've encountered.

People with omhalophobia are terrified of bellybuttons - usually touching one or having theirs touched, but panic can set in even after just seeing a picture of a navel. Some individuals break into a cold sweat just thinking about accidentally eating a dish that contains garlic. These people suffer from alliumphobia (from the Latin name for the onion plant genus).

Chromophobia is the excessive fear of colours. Sometimes that means only one specific colour, such as red or purple, which are named erythrophobia and porphyrophobia respectively.

It also turns out that not everyone loves sunny days - people with heliophobia are terrified of the Sun, or sunlight in particular, and will go to great lengths to avoid exposure to it. Likewise people with aquaphobia don't want anything to do with water, even though we are all largely made out of it.

But perhaps the strangest phobia we've heard of is sidonglobophobia. It's the extreme fear of cotton balls - usually these individuals can't touch them, and are particularly terrified of the sound cotton balls make when torn apart. We weren't able to find any accounts in clinical literature, but you can certainly find flocks of sufferers in online communities.

According to Nick Haslam, there are many other strange fears: "The clinical literature records phobias of rubber bands, dolls, clowns, balloons, onions, being laughed at, dictation, sneezing, swings, chocolate, and the wicked, beady eyes of potatoes."

I don't know about you, but a devastating fear of chocolate sounds heartbreaking. Thankfully, these days cognitive-behavioural therapy has excellent methods to combat phobias, so most people are not stuck with their cotton ball horrors for good.